68 Dodge Dart engine swap??

The slant-six transmission will not work with any V8s.

340s are cool, but a 360 would be the same amount of work, for less money. Unless you already own a 340, that is.

There is no real advantage to changing the k-member, unless you go with a 73+ unit. The driver's side mount is actually the same on the slant six and small block k-members. It's the passenger side that is different.

To swap in a small block, you'll need different motor mount brackets, a different transmission, it would be a good idea to get a bigger radiator, and you'll probably want to swap in a stronger rear axle. If you put in a different axle, you'll need to shorten your driveshaft.