I have a 1973 Olds Omega (Nova) clip under my P15. We narrowed it 3/4 of an inch. It drives like a dream, but the car wasn't drivable when I bought it so I can't compare.
Disc brake and rack kits were not available when my buddy and I put it together. I could not have done it by myself. Jerry had experience with the Nova clip swap on a couple 46 Fords (narrowed 1 inch) and didn't have any problems getting it under the Plymouth.
With the stuff out there today I'd probably go the upgrade route now, if I were doing it myself.
Just a lot of things can go wrong if you don't have the experience, or an experienced buddy. I understand how half done jobs end up parted out.
As a side not, Jerry is now driving a 46 Ford with a MSII front end and has said several times the Nova clip drives a lot nicer.