Are you talking about the long curve looking down on the bumper as it curves around the front of the truck or the short curve in the bumper looking at its end??

I do not have anything between the flat bumper bracket and the back side of the bumper. My bolts are tightened to maybe 50 foot-pounds, the lock washers are completely flat and there is no deformation in the curve of the bumper. I'm wondering if by using the original or repop bumper bolts that have a slight curve behind the heads may help retain the short curve of the bumper when tightened.

As for the long curve I had to add 2 or three washers between the brackets and the truck frame then tighten all of the bolts a little at a time to get all to fit sorta correctly

1941 Chevy Master 3/4-Ton
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Born to fish, the truck is only a hobby.