for the optic mount you can weld a rail to a machined down RSb but if your going to get serious about building a target set up you really need to go bigger on the OD and that requires more work. as you will need to mod the barrel bore onthe bottom of the block and then machine the top off to take a rail and to be honest the rail needs to be thicker to do a canterliver set up but it will work. i machine my rails from scratch. the other route that is not as pretty is to weld in a steel bar and then attach a aluminum rail on top of that.
On my kids hunting build I wanted the scope low as possable so I milled out the under side of the rail so the cover would still come off with the optic rail lower.
if you dont hand load there is no sence making a custom mag and mag well for seating bullets out further as you cant buy them like that unless you custom ordered them from some custom ammo maker.
a 20" or so heavy barrel fast with a twist will let you shoot 69 grain match kings well and you can try the hornady 75 grain stuff. it will also shoot 50 grain bullets well
( mine does.)