I don't really see where you are gaining here. If you keep the standard horizontal cone pulley drive you are stuck with 18 to 20" bench width to fit the cone, useable belt length and adjuster in. Given that its probably easier simply to dump the standard Vee to flat countershaft drive, with its irreducibly big driven pulley, and either use a polly Vee (serpentine) or a two stage plain Vee belt drive. Single or two speed, all new or modify the existing casting, to taste. Either way the large step of the cone pretty much defines the bench depth. I've done the two stage, two speed Vee drive several times over the years on both SB and other breeds, works well and is easy to include a jockey pulley type clutch. Can do you a suggestions PDF if you like. Nowadays I'd seriously look at the poly Vee drive, still with a jockey pulley clutch, even if you don't exploit the clutch the spring loaded jockey pulley tensioner gets you out of the matched pulley & belt length trap. especially as you plan to use a VFD so speed trimming is easy.
From your reference to over-speeding I guess you are looking to replicate a two speed system with a simpler drive. Standard SB countershafts tend to run in the 390 to 410 rpm range so a 4, or 4.4 to 1 reduction would be a better fit. 1400 rpm is about as fast as I'd care to run a segmented type cast iron bearing without a bit of attention to lubrication. Push to 1,600 maybe if the wicks and all are on top line. Only reason for seeking such speeds is real tiddly work or to properly exploit carbide tooling neither of which is really 10" SB forte. To get this sort of speed you will need to double or more the standard motor RPM using the VFD. Do-able if you have a designed for VFD motor and its matching drive but risky with a standard motor. If you are expecting constant output power you are in drive systems engineering territory. Most VFD units have input power draw limits but how much actually gets through to the shaft is a different matter.
There are very good reasons why the "always safe" variation on a reasonably modern motor is considered to be around the ± 30% region and on older ones ± 25 or even ± 20% is more prudent. VFD units tend to supply the power needed so at part load the safe operating area extends considerably. The problem is that motor efficiency drops badly once you go too far beyond the design rpm, depending on about a million constructional and design factors, and Home Shop Guy has no way of measuring what is actually going on. Falling efficiency means higher current draw for the same torque. Higher current means more heat inside. Too much heat evaporates the magic smoke!
Clive