Nothing wrong with having a spare motor. I got 2 for now. a 72 400 and 70 340. the 400 I would get rid of, but the 340 you need to get from my grave!
The BB swap is pretty involved. The SB rad will not work because of different inlet/outlet locations. The k member will work fine. as...
Hands down the 340 is the better way to go. It will drop right in and need almost nothing. The 440 will need as mentioned a whole laundry list of items. Besides the 340's lighter weight will make the car handle better and getting 400+ from a 340 is very easy to do.
As for plug changes...
There is NO way that a set of 452 open chamber heads has a cc of 79 unless massive decking was done which means the intake face would have had to been modified too. 383's are notoriously over advertised on compression and need considerable deck work to make right. The online calculators are...