My 48 Chevy frame had stenciled on the inside I presume from the factory something to the effect of "do not heat or weld on frame"
This same message is also found on many if not all large truck frames today.

I would avoid welding on your frame.
I would also not want to box it not only because it involves welding, but because it changes how the frame acts. It was designed to flex, boxing it in places can lead to a cracked frame at the transition from boxed to open.

Grigg


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-