2 suggestions.. use a larger tube and piston , maybe in the .750 id range , this would equate to more swept volume per pump , but the pumps would be harder. other suggestion is to make it a sidelever , this will allow a longer pump arm bringing the effort needed back down to factory ( or less.
How do you physically make the valve body without having a joint in it?....
It looks like you have moved the transfer port way back to gain volume?....
Won't that drastically reduce it's volume, however?.... Perhaps not, if the check valve end was threaded in, but even then, wall thickness could be an issue, and require boring out significantly larger than the threads....
One thing you might consider is a "moving O-ring" style check valve, where the stub it sits on (which normally moves as part of the check valve) is threaded into the front of the valve, using a flat-head screw with a tiny hole through it.... Only the O-ring moves on pumping....
A 12" barrel that is .30 cal would be about 14 cc.... so 4-5 cc would seem about right, I would think...
Through the sides of the tube would make sense, not in the way then....