You may consider bolting the flanges together tight at the rear. And bolting them together with a good stiff compression springs under the nuts on the front end. You sometimes see this done on longer wheelbase trucks when mounting the body. This allows the truck frame to flex indepentely of the body frame.

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-