My 340 in my 66 developed issues :-(

OP, slow down and don't panic or do anything rash. I would not want to cut the inner fenders either. Check out the Layson's Spitfire header reproductions that are made for the early A body cars. Check on delivery time if you choose this route; Layson's can be slow at times.

LAYSON'S RESTORATIONS - DODGE, CHRYSLER, PLYMOUTH RESTORATION PARTS

Pishta's suggestion above is another good route.