Almost everything from the '76 Dart Sport will fit your '72 Swinger. How much you want to switch is up to you.
If you're converting from a /6 to a V8, the easiest thing to do would be to use the entire K frame assembly and suspension from the '76. Using the '76 K frame will get you V8 engine mounts which will be the later spool type engine mounts, the UCA's will be the big ball joint uppers that work with disk brakes, and I would imagine that the '76 probably has disk brake spindles etc already. If either car has a sway bar, make sure you switch to the LCA's that match the '76 K frame, as you can't use an 67-72 sway bar on a 73-76 K frame. All of the suspension will be bolt on.
You don't need to, and wouldn't want to, switch the entire front clip (fenders/hood/grill/bumpers) as the '72 already has by far the more desirable front clip. Just swap the K and suspension. You'll also need the radiator for sure. The /6 wiring can be modified to work with the V8. You could swap the entire engine harness over, but that will get complicated because the '76 engine harness isn't a direct swap onto the '72 bulkhead connecter, you might have to move some wires around. Or use the entire '76 harness, engine and dash. The steering column will swap over, and since I'm assuming the '72 is a column shift and the '76 is obviously a floor shift, you'll want to do that. The only snag will be if one car is manual steering and the other is power. If that's the case, you'll need to keep the steering shaft matched with the type of steering, or swap the steering box as well. But swapping the steering shaft out of a column is pretty simple.
If your '72 isn't a 4 speed car, then the entire operation will not be bolt on. You'll need a 4 speed transmission hump, which will require cutting the floor of the '72. You'll also have to slightly modify the 4 speed hump from the '76, because the floor on the 76 has a raised section to clear a catalytic converter that the '72 does not have. The ball stud mount for the Z bar should also be welded on. The rest of the parts bolt on and should swap right over.
You may want to check what rear end is in each car before you switch them. The '72 probably has a 7 1/4, and the '76 probably has an 8 1/4, in which case you should definitely swap them. Even if they're both 7 1/4's, you should swap them, because the '76 will be a BBP 7 1/4, and none of those parts will exchange with a SBP 7 1/4. On the off chance the '72 has an 8 3/4 (unlikely for a /6 car) you'd want to keep that regardless of bolt pattern.
The Dart Sport's driveshaft will not work in the '72. The Dart Sport has a 108" wheelbase, the Swinger has a 111" wheelbase. So, even if you swap the entire drivetrain, you'll still need a new driveshaft.
Other than a few parts of the 4 speed conversion, everything will be a bolt on. That doesn't meant it won't be a lot of work. I just finished switching my '74 Duster, which was originally a /6 with an auto, over to a 340 4 speed. You have all the parts, but it will still take some time.
Here's a thread to my Duster build.
http://www.forabodiesonly.com/mopar/showthread.php?t=198098
Not everything is covered there, but there are A LOT of threads on /6 to V8 and auto to 4 speed conversions on here.
I guess the obvious question is, what's wrong with the '76 Dart Sport that you don't just drive it instead of swapping everything over?
Good luck, and welcome to FABO! :thumbup: