Offshoreonly.com - what needs to be changed gen V to gen VI

ok pulling my blower motor to put stock power and drive back in my boat, noticed my stock motor had popped a freeze plug so i repaced it and ran it out of boat to check for scracks in block and sure enough within 5 min my oil was twice as high as it should be. I located a few gen VI blocks from a guy but want to know what needs to be changed over. The guy with the gen VI merc racing 502 blocks said they are set up to run roller or standard cam. I thought gen VI blocks were all set up to be roller only. what else needs to be changed besides, gaskets, rings, and bearings? what else is diff? I also have a 454 mag motor i got from a buddie with only 20 hrs since rebuild who also cracked a block, am i better, off just rebuilding the 454 then switching to a diff gen 502 block. I can get the 502 block for $900.00 honed and ready to go or i can get a 454 block just magged ready to rebuild for $500.00 and i would not need to replace anything but gaskets and rings. Is there any vale in selling a almost fully dressed 502 with 125hrs and a cracked block to someone who wants to do the work themselves? Any and all advise would be great...