440roadrunner

!!!HUH!!! I didn't even know there WAS such a thing as a '65 GT
I think you'll find your Mopar experience very similar to Ford.
Check out local clubs
Check out car shows and swap meets
eFray
"Year One."
One thing you may not know, is that there is one HELL of a lot of difference between an hemi 'Cuda and say, a starter car which had a slant 6 or 318
For one thing the "bigger" 'Cudas had bigger rear fenders --at least the tire cutouts. I don't know what all versions had this, certainly the hemi and probably the 440 and 440+6 cars
Otherwise, on "lesser" cars, the brakes, suspension/springs, rear end, were all of lesser "duty" than a hi performance version, so you can plan on "gutting" the bottom and redoing the whole suspension.
If you got an 8 3/4 rear, those will do if you don't go completely nuts. A well-built posi 8 3/4 is probably not QUITE as strong as the Ford rears, that's up to speculation.
If the car came with a 4 speed, you can probably use that. The 440-6 and hemi 4 speeds had 18 spline input shafts and were somewhat heavier, but the smaller 23 spline units are still pretty strong. Two transmissions to stay away from--and I don't know how late they used these--is the 3.00-3.09 first gear units used in some slant 6 and 273 cars. These smaller shafts and were weaker. One other to avoid like the plague is the stupid (and Ford tried this too!!) so called overdriven or overdrive 4 speed. What Mopar (and Ford!!) did was this: They re-geared the 3rd gearset so that it was overdriven, and then simply turned the external shift lever on the linkage UPSIDE DOWN!! In other words, when you put the shift lever into "4" what you REALLY did was put the transmission into the 3rd gear position--using this overdriven gear. These were horrid transmissions for both Ford and Mopar, and like the 300-309 geared 273 units, had smaller shafts
If the car is a smallblock, and had an auto, you can't use that--the bell pattern is different from the BB engines. Of course if it had a stick, you also can't use the bell
If you intend to save a little here and there, I'd advise getting at least a '383 car. that way, you can have the transmission rebuilt, and all the linkage and mounts should be OK, except for the K member. I've heard that 6 cylinder K members can be used for a hemi swap, but I'm not sure.
Which of course leads us to that big issue--the hemi cars used a special K member--the big crossmember under the engine which supports the engine and suspension. I don't know if you can modify other K members or not. I did once put a 440 into a 'Cuda which originally had a hemi, but I had to weld new mounts. That was a long time ago. I don't remember the details.
One thing I do NOT know and would pay you to check out, is whether the transmission (floor) hump was the same in all these bodies.