38-55 will give you excellent range and accuracy + low pressure. I have shot 45-70 for quite a while and it too will do a fine job with a little work.

All- I did the math and if you buy two of those laser bore site devices for your bore, you can save at least 200+ rounds in the regulating process.This approach will get you VERY close
Build a fixture to give you small movements of the barrel. go to a range ( at night is best) place your targets out to 50 yards. plave you barrel upside down towards the target, and then adjust the barrels accordingly. Then clanp the barrels for welding/tig/epoxy/etc.
The rest of the job of course is dependent on loads.........Thought??
TC