There was a company, I think it was called Thoroughbred, or something like that, which was mounting fiberglass Ford pickup bodies onto Ranger frames. Not sure if there body was the same as an original or not, and I seem to remember that the company had a really bad reputation.
Anyway, they used the older Ranger frame, which had the twin I-beam suspension. That suspension was sturdy, and dependable, but not really what you'd want under your truck for handling and ride.
There are much better options that will give you better ride and handling. 35-40 frames are easily found for under $300.00. Most people just upgrade to the Mustang II type suspension. You are the first I've ever heard that had a bad experience with a Heidts Mustang II suspension, are you sure it was installed correctly?
Well, no matter what you do, I wish you luck. I'm just starting on my 35 pickup myself.
Later - Karl.