Hmm, double acting is going to be a little tricky. You'll need to weld a fitting on the rod end of the cylinder. That'll warp the tube. It can be threaded into the cylinder, but only if it's a heavy wall tube.
Maybe a spring return within the cylinder? That will take a bit more length. Drill and tap the butt plate for the pressure side.
Tie rod jobs are just more work for no gain in this app.
You can buy polypak pistons from suppliers, (al. or ductile) or make your own.
As for the rod end seal or head, there's ductile threaded of all types. On this job it would be more work than necessary. I've made many aluminum ones that fit into a counter bore and retained with a snap ring. Some weren't even U-cupped, but just an O-ring seal with a wiper at the end. If it was spring returned, there would be no pressure there anyway.
The piston can be made of al. as well. O-rings can be used there too. I like a U-cup for the piston seal, but there's a lot of O-ring units out there that are working their little guts out. Four inch is usually the higher limit for an O-ringed piston, but have made larger. Much larger for air apps.
I use Scot Industries for rod, tube and ductile. They are the absolute best in my estimation.
Cylinder Repair Components, Hercules Sealing Products, Bailey's and B-D Cyl. Products handle many parts.
