Magnum heads do work on LA's. The valves are bigger and might not like 273's small bores. I don't know for sure. I haven't heard of anyone putting Magnums on 273's. There's other things to change to make them work though. Magnum rockers oil through the pushrods so you need different lifters (on a hydraulic cam), pushrods and the like. The intakes bolt differently as well so you will need a different intake. It can get pretty spendy if you are on a budget.