I may be misunderstanding something, but I don't think the Galaxie (full sized Ford) ever used unibody construction. It may have different spring towers than some other cars (I don't really know) but it has a full frame underneath.
If you aren't stuck on using an original vintage engine, why not try a complete 4.6L engine out of a 2005-20xx Mustang GT? There are quite a few in the wrecking yards, and if you pull the complete setup you have a 300 HP engine that is smooth and reliable. If the Ford Racing supercharger fits under the hood, you can also bolt on at least 100 HP without any problem.