There are a few threads about boxing frames, and my take is that it's asking for trouble. And or for the work involved it's not worth the actual gains and problems created.
Perhaps it's OK for a frame that was designed form the start as a boxed frame, but welding on a C-channel frame to box it without a complete understanding of welding, required frame flex, and other factors may not be such a good idea?

I did effectivly box my frame just at the rear engine/transmission mounts. Made matching C-channel pieces that bolt inside the frame, helps transfer the weight of the engine on the top flange down to the bottom flange as well, the rear front Spring mounts also bol through this piece. And the crossmember tying the frame rails together bolts to them as well.
this picture shows the two boxing pieces (one front left on trailer) and the new U shaped crossmember [IMG]http://thumb9.webshots.net/t/59/59/7/77/48/2858777480080251109QvHMaJ_th.jpg[/IMG]


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-