The only diffs I can determine between a LM-1 and a L-48 are:
L-48 has 10.25 compression heads; LM-1 has 9.0. Same valve sizes.
The L-48 has 4 bolt mains; LM-1 has two. L-48 has dual exhaust...LM-1 single.
Duals and the higher compression make up the 45 hp difference. Both engines share the same cam, pistons, rods, crank, block, manifolds, carb, etc. They do employ different gas filters. But the LM-1 rated at 255 wasn't a bad little engine. It also looks like most LM-1 cars got 12-bolt rears with Muncies and Turbo 350's. So, your LM-1 could be a really nice performer with a little help.