Did you ever try multiple shots without pumping? I welded a tube together with a full size valve and 2200 pump arm and could get 4 powerful shots before I got a weak one after I adjusted it so the flat piston was touching the valve face at the bottom of the stroke. I didn't have a chrony to check velocity or Eva to adjust either.
I just welded up the holes on a 2100 tube and added about a inch and a half to the length so I could have a full sized valve and used a early 760 power master valve front (already flat). How did you attach the adapter and gauge? That sounds a lot like what I'd like to do!
This part of the project is probably done... hoping anyway...It's sometimes trial and error to get the spring length right when using an SSS...