Grigg, my frame is the same as what was used under a 4100 with a 14,000 GVW. Its dimentions are 7 x 2-3/4 x 7/32.
Regarding crossmembers... Yesterday I removed the bellhousing crossmember from a 1-ton frame I've been cutting up to use for brackets... Three of the four tabs on the crossmember where it is riveted to the frame sides had broken off from the crossmember.
So yes, a stout crossmember is part of my plan. Current thinking is adapting the transmission crossmember from a big block 3/4-ton pickup. I have that in my scrap pile and it looks adequate.
Now that you mention it I see/remember your point, the 4100 frame is all but the same as a 3800, but with a 5,200 lb higher capacity. Let's assume the frame is well suited to the 4100, and way overkill for the pickup, you should be fine.
When I started this I wanted a 137" wheelbase and big fenders, so my choices were 4100 or 6100. Decided on the 6100 because the stronger frame sounded like a good idea with the heavy diesel and 10 speed transmission, even though the 2 ton frame weighs more, and an overall light weight truck is a major consideration.
Ended up with a 6400 and actually prefer the extra 2' of bed now that I have it.
The original crossmember I removed from my 6400 had a few cracked ears too.
Apparently the original ones were none to strong, obviously something even more substantial is in order.
I'm sure you've investigated the U shaped crossmember from a medium duty truck Like I used, what are your thoughts on that type and your build?
Grigg
