forgive my for not reading all the posts completely and maybe repearing something, a lot to read but here's my two cents.

The biggest ford Engine I believe is the 460. It will fit with some work and modifications, we have a member on here who did it. Or get a 351 and stroke it to 408 for about $1,000 to stroke it alone, besides rebuilding fees, machining, etc. This is an expensive part of the car. The 289 and 302 are very versatille engines. The 302 is easy to go to EFI becuase it's essentially a 5.0. The 351W and C are very popular becuase they are big but still fit without modding any part of the car's body in any year. I prefer the 351 just because I do,lol. You can try a 67/68 with an FE block of sometype or a Mach 1 from 69 with a 428 or if you have deep enough pockets find a Boss429. As for 65, like said, the biggest then was a 289. Good engines but for a large amount of horses, you'd have to completely overhaul the engine with new parts all around, maybe bore it, flat-top pistons, new cam, new intake and carb, HEI igntion, and some roller rockers to top it off would most definatly get you some horses but its once again gonna cost a pretty penny. Good luck with the decision and we're here so feel free to keep asking questions.