[bigblockv6]

I had the heads ported and polished on my 478m during the build, of course the Magnum engines already have larger intake & exhaust ports as well as valves over the non Magnum engines namely all 305 and 351C engines. Don't know which engine you have but if it's a 305 using Magnum heads will actually lower the compression on the 305. You may just want to have the 305 heads ported and polished and have larger valves from Cat Diesels installed because larger Magnum exhaust valves have hollow stems filled with sodium that are notorious for snapping from over revving the engine.