Mt. Jim, you could try an instant glue from Lok-tite to hold roller pins in place, it takes very little heat to melt the glue if you ever need to take it apart.
If you don't mind ordering from Europe, Fumotec offers cylinder kits in various bores, you can custom make any length you want. My big track hoe, a little bigger than yours, uses a 20mm bore cyl on the stick and the piston rod is atleast 1/4 inch thick (I'm guessing) I've run pressures higher than 5mpa, I have never bent a cyl shaft... the seals in the valves however can't take such pressure for long, try to stay under 4mpa.
https://infopro-en.fumotec.de/hydrau...erteilesaetze/