I don't think boxing a frame is such a good idea in this case.

It complicates the instillation of lots of other parts, wiring, brake lines, trailer hitch, anything that was bolted to the frame is now harder to do. You could cut large oval or round holes in your boxing plates to allow some access, but still a lot of work for some added complication.

I'm concerned that you will be removing the transmission crossmember which does a great job of tyeing the frame together at the rear of the front springs. To remove this would add some stress on the rivets holding the front crossmember in, as well as allowing the frame to try and twist and possibly spread at the rear of the front springs. Adding a crossmember a ways back will help some, but I don't see how boxing the frame will help as much as a properly placed crossmember.

Take a look at my solution to the same problem.
http://rides.webshots.com/photo/2236622090080251109tywgjR
I removed the transmission crossmember. Then fabricated two short sections of channel to "box" the frame for about 10" or so. The channel is bolted through the frame top and bottom with the spring hanger holes, new motor mount holes, and some extras as needed. Finally a new U shaped crossmember is installed in essentially the stock location to tie the frame back together and be low enough to clear the engine and transmission.

Something to think about,
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
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-