I'm thinking you should just go ahead and drop that Hemi into the GTX. Or lift it in from below if that works better. Life is short, summer is around the corner so you might as well get it bolted in and start driving it. You can deal with the patina later, or just not deal with it at all. These days I think it is perfect to have a muscle car with some patina and a 572 in Hemi thumping away. That car will get a lot of attention once you start driving it. You do want to make sure it is safe but other than that I'd say go for it.