Is the stock front suspension gone or toast?
If not then I would leave it alone and spend your money, time and effort elsewhere. These old Mopars had pretty good front suspensions and were far ahead of the Fords and Chevys of the day. All it really needs is a stock rebuild with readily available parts, a rustyhope.com disc brake kit, and relocate the upper shock mount to the frame with some simple brackets which rustyhope sells as well. If you want to lower the front, Fatmans sells dropped spindles and a rack and pinion steering kit.
I am on my second 48 Plymouth Coupe. The first was a Business Coupe that I had professionaly subframed from a Nova. Ran a big block Mopar 383 / 727 combo. My second I decided to stay with the flathead and stock front suspension. I can tell you for the money and effort involved with doing the subframe, I wish I had never cut the frame on the first one.
Unless you go with a totally new aftermarket front suspension you will most likely have to rebuild anything that you would graft onto the frame anyway. So, why not just do the rebuild on the stock independent front suspension? With the discs they will stop about as good as any suspension you would swap to and with the shock relocation it will ride about the same as well.
If going with a V8, all you have to do is move the engine about 1.5 inches to the passenger side to clear the steering box and you are good to go. Several Mopars from the factory were built with the engine offset like this during the muscle car era with absolutely no problems. You can do the same with the 48 Plymouth.
I would also suggest that you check in over at the P15-D24 forum www.P15-D24.com - Powered by vBulletin
there is a lot of knowledge on the late 40's Mopars over there.