For the longest time I had gasket sealing issues running LA headers on magnum heads. The ports are different enough where the gasket is thin is some places. I found that using Remflex gaskets (THE best gaskets I've ever tried, and I've tried a bunch) to match the header (LA) fixed the issue. They're also thicker than standard gaskets so they push the header away from the block a bit. Not sure if it'll be enough to clear the magnum motor mount bosses though.