The front suspension from the '71 can be adapted to the stock '51 frame. You need to make some spacers to fit between the frame and the crossmember, and between the steering box and the crossmember, to make that work. And you still have the track width problem.
Plus, the rear from that '71 will be too wide under the back of your '51.
You'll be looking for offset wheels for both ends.
Provided you aren't worried about the track width issues, I'd rather adapt the front suspension than shorten the frame and build all new mounts, but that's just my opinion.
