I've never had oppotunity to study a 4100 frame but the specs suggest the only real difference is the bumper brackets. A major compoment of weight ratings appears to be brakes.
Look the the differnce between a HO50 and a HO72. The load rating really comes down to 12 inch versus 14 inch brakes. The rear axle on a 3800 versus a 4100 is a whole different class with far more differences, but the brakes are an important part of the higher rating.
I studied your u-shaped crossmember when you first posted it, and then looked at one in a wrecking yard when the opporunity presented itself.
For my build it doesn't really make sense. My engine transmission combination does not use a mount in that area and in fact the crossmember interferes with getting the motor low enough in the frame.
I've removed that crossmember from my frame. It was not broken on that one and I sat it aside in case I ever need one.
As I began this build I debated between IFS and straight axle, and it turned out to be that crossmember that finally settled the issue. You need the crossmember between the spring mounts on a straight axle truck. You don't need it on an IFS truck unless you are mounting engine/trans there.
The IFS donor is a '72 Chevy C20. It did not have any crossmembers or boxing between the IFS and the transmission crossmember. Since the frames are similar in design between the crossmembers and I'm using similar power train I believe I'm okay with this approach. (The C20 was a 402 with TH400.)
I have thought through a couple of approaches to creating k- or x-members through this area, and once I'm putting the trans crossmember in place I'll make a decision on that.
