Moving it to the Marlin LA is probably a good move. Those throaters cut so easy that going too far is just as easy.
The .008 cut off the breech end am assuming is to allow the Marlin to headspace (their bolt heads are so much deeper). If it were me I would only remove enough to allow minimum bolt head clearance (<.005 ?). I believe Marlin protrusion is .140 or so.
Only .008 or so can easily be removed and squared with a bastard if a lathe not available. The web end of the case should not be expanding at that end of the chamber so getting it within .001/.002 square will never be noticed. Make sure to add a slight chamfer at the case mouth and polish that edge free of burrs.
Also any final adjustment to increase the headspace can be also done without a lathe with your reamer & a tap handle.