[monocoque]

Let's try this again...with a few added details. After some measuring.

11 cross-members at 61 inches each will require about 60 feet of material. 23 outriggers at 14-3/4 inches will require about 30 feet of material. And two 29 foot main rails will require about 60 feet. So the total material is 150 feet of steel.

Using 5" channel, 1.750" flange, .190" web thickness at 6.7 lbs 150 feet is a 1,005 lbs frame. The original frame probably weighs somewhat less than 1,005 lbs, although the main rail material is likely the same, because of the lighter factory outrigger and crossmember material.

Using 5" inch channel with a 1.885 flange and .325 web thickness at 9.0 lbs per foot 150 feet of material is a 1,350 lbs. frame.

Using 5x2 rectangular tubing in 3/16 gauge at 8.15 lbs per foot of material 150 feet is a 1,222 lbs. frame.

I'm guessing, using either of the heavier materials, channel or tube, I'm looking at a frame approximately 500 lbs heavier than the original frame? Here again, in terms of weight, the difference between channel and tube would seem negligible.