63 dart v8 swap

If you use a 727 tranny, you will have to beat in the lip in the trans tunnel for room. You need a new driveshaft regardless, since your tranny will probably be the yoke type and I think your wheelbase is shorter. Most people upgrade the rear end, which usually means BBP wheels, so change the front end to match, or carry 2 spares. And your shift linkage will be very different, so start designing.

To summarize, as people say it is very easy - just change the entire drive-train, front and rear suspension, and major parts of the steering, plus cooling system and you are almost there. A lot of work to add 2 cylinders. Or you could turbo the 6 and be really cool, fast, and efficient.