Guys I found a nice 74' 240D with a rusted floorpan, other wise nice condition and a 78' 240D with no rust just needs radiatior. My question is, can I take the body and radiator from the 74 and put it on the 78 chasiss? I really like the older body style. Both cars are only $350.00 combined?

These cars have no chassis..... if you mean you want to un-weld the uni-body then its more trbl than its worth, you might as well just repair the rusted floorboards on the older cars or scout around for a better body in the style you require.
The front of the older cars (chassis W115/114) have a subframe to hold the front end together. The newer cars (W123) have their suspension bolted directly to the body...this alone would cause a major headache.

The only thing that is interchangeable is the engine and possibly gearbox/diff but not the driveshaft.

Anil