I can understand your wanting to use stainless steel. It is misleading as it is NOT as strong as carbon steel for bolting. It also has the nasty propensity to gall (cold weld) when stainless nuts are run onto stainless bolts, or stainless male pipe threads are screwed hard into stainless fittings. This is well known and something industry has dealt with for ages. Whenever possible, if we have to use a stainless bolt (as we do on certain application on hydroelectric turbines), we use a stainless stud or bolt and a Silicon Bronze hex nut. The Si Bronze is every bit as strong as the stainless, and being dis-similar, will never gall. And, it is galvanically inert or close to it, so that is not an issue. We NEVER screw stainless pipe fittings together dry. We always use some sort of pipe dope. Teflon tape is bad news around hydraulics as shreds of it have a way of migrating into hydraulic systems and winding up inside control valves and similar. We use paste-type pipe dope, carefully applied. Pipe threads are a sharp vee, and are known as "dry seal threads". They are cut on a taper and "wedge" and the sharp vee assures very nearly full flank contact on the threads. This close fit and "wringing together" is a recipe for galling with stainless steel on stainless steel. Pipe joint compounds are as much a lubricant or anti-seizing agent as they are a sealant. In theory, the pipe threads should make up to seal without any help from paste type sealants or Teflon tapes. We use "Swagelok" ferrule style tube fittings quite a lot in powerplant work, including for hydraulics. In stainless, they are quite pricey. Swagelok uses slightly different alloys of stainless (like 304 vs 316) for the gland nuts and fittings. Since a ferrule is used for the actual seal, Swagelok uses a slightly looser fit on the gland nut threads of their fittings. Otherwise, they would gall. I've made the mistake of "dry fitting" stainless pipe fittings and nipples together to try a fitup, no pipe dope or lubricant. Hand tight, the fittings galled and that was all she wrote. We got things apart with heavy wrenches, but the damage was done to the threads.
As for bolting in structural applications, I would NOT use stainless bolts. It may look pretty and not corrode, but you are running them in carbon steel frame and similar. I'd use grade 8 or A-325 structural bolting, made up with an anti-seizing paste on the threads and faces of the nut and bolt head and call it done. The stainless bolts are not up to the job of holding something like a splitter wedge or ram guide shoe together. No need to spend the extra money on stainless. IMO as an oldtime engineer, it is the wrong material for the application. It may not rust, but more importantly, it may not withstand the stress cycles and loads that carbon steel bolting would in your application.
If you use stainless pipe, it does not gain you anything over carbon steel other than corrosion resistance. The issues with close nipples and thin wall and stress risers are still there. If you use stainless steel screwed close nipples, go for Schedule 80. Same problems with breakage across the threads in applications where side-loads, stress cycling and vibration exist.
Most logsplitters I have seen (and one I built for friends years ago) were meant to do work, not look like they were spit shined for a show. Most are built of carbon steel, often as scrap of whatever people can make off with from jobsites. If they get a coat of Rustoleum gloss black or red, it is a step up. However, in terms of structural strength and mechanical design, they are right up there. A-325 structural bolting, welding done with stick using E 7018 by people who have generally welded in the power plants or building trades... piping run using 300 lb Black Malleable Iron fittings (this 300 psi is a steam rating, cold fluid rating is much greater) or screwed 3000 lb forged steel fittings and sch 80 nipples.
Stainless steel has its place, but quite honestly, it is not the best application to put it on a wood splitter. It is not overkill as it is not equal to sch 80 carbon steel pipe or carbon steel structural bolting. I would NEVER use stainless bolting on the splitter's axle mounting or trailer tongue if you plan to tow the splitter on the public roads.
By way of example: about 40 years ago, I worked as a young engineer on the construction site of a nuke power plant. We had to dismantle some large check valves to flush through the piping. The bodies of those check valves were cast stainless steel, butt welded into heavy wall stainless piping. When we opened up the valves, I was surprised to see the internals of those check valves were bolted with carbon steel B 7 grade stud bolts and class 2H heavy carbon steel hex nuts. I remember asking an older engineer whom I considered my guru about it. He explained that stainless steels were not the wonder wagon that too many people hitched their stars to. He explained that the B7 bolting and nuts were used inside the valves as higher strength was needed, and they did not want to screw a stainless stud into a tapping in a stainless valve body and risk galling. It was then that I was made to think in depth as an engineer, and started to think in terms of "the big picture", looking beyond the major "attraction" of stainless steel (corrosion resistance).
A splitter is a piece of outdoor power equipment. It sits outdoors, perhaps gets rained on, and issues with dissimilar metals (like aluminum parts bolted to bare carbon steel) might prove to have electrolytic action over time. But, a splitter is meant to be used. In short order, the paint gets worn off the beam, and there is wear and tear on it. You go out to split wood, and you may be working alone, heaving hefty log butts into the splitter. A splitter gets used and takes a bit of a beating where I come from. People use old hydraulic cylinders from junked heavy equipment (typically yellow), old structural steel for the beam and ram guide shoe (black), and beer kegs for the hydraulic fluid sumps, or they put an old beer tap handle on the valve for ornamentation if they are going out to build a show-grade splitter. The engines are usually having the paint burned off the mufflers and are grungy before too long. I've never seen a wood splitter look new and bright and shiny for too long if it does any serious splitting. Build it to use it and build it to last. That, and how it holds together and splits wood are what counts. Looks are secondary. Who looks at a wood splitter for "looks" out in the woods ? We look at the size of the ram and the engine and pump and how it's built and remark as to approximate tonnage or how easy it is to load log butts onto. Your back will be the ultimate judge of how good a job you do of designing and building a splitter at day's end.