The main difference is the oil passage to the heads is not there. You can drill it out, but you would have to be very careful that the oil passage is not blocked by shavings. Personally if it were me, I'd just use the Magnum heads and all. If you are running a carb, you can pick up a dual plane 4BBL intake for the Magnum and used that. It would be a considerable performance increase over your LA 318. Use the timing chain cover off your LA 318 and everything outward from there and you can use all your own brackets and pulleys. Or you could adapt the Magnum serpentine belt setup though it would be a little more work.
Depending on what you are paying for the engine, even with a new $300 intake manifold that could be the best spent money for the power increase that you are likely to find. It costs a pretty fair amount in parts and labor to bring a stock '80s 318 up to the power level of a stock 318 Magnum with a 4BBL carb. If the Magnum engine is healthy, you can be sure that you will probably come very close to doubling the horsepower of your current engine. There may be some sacrifices to do the swap but it's a pretty decent gain for just an engine swap with a few bracket issues and a new intake.
The Magnum heads, if not cracked, are WAY better than the factory LA heads. If the heads are good, I would buy an intake manifold to run a carb on the Magnum engine. Mopar sells a pricey one, but there is a Chinese knockoff of the Performer RPM called the "Crosswind" that is drilled for both heads and is very cheap.
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