You have a burnt out exhaust valve.Depending on whether it has sodium filled stems or not, that could be a fairly cheap fix, the rest of the engine shouldn't need anything done to it.
The 391 is a heavier casting than the car engines,but the only things that won't interchange are the distributor and oil pump and drive, they're bigger on the pump drive. The bushing hole in the block can be drilled out so the 391 pump and dist. will fit in a 390.IIRC its a 33/64 bit.