If you're going to move the rear axle forward....why didn't you just extend the frame behind the axle? It would result in the same frame length, and you wouldn't have compromised the frame strength between the axles (where the load is placed).
I'm hoping you put a 1/4" x 4" piece of flat steel or C-channel in that frame rail - I've lengthened a few vehicles...and you HAVE to reinforce the inside of the frame after doing the weld-in of the new outer skin.
The main reason for a sub-support is that you've made the exterior welds pretty by grinding off 60% of the weld. They don't grind off the welds on super skyscrapers and other major metal structures for a reason - that's 60% of the strength gone once you grind it flat -you're depending on the penetration and metal bond inside of the gap you cut in the first place to hold the frame together now. I hope you have 100% weld bond between the frame sections!
You're certainly onto something quite unique here...and would rather see it run solidly for the next 40 years as opposed to bending in the middle due to stress and ruining all of the awesome sheet metalwork that you've done.
Not being critical, just my $0.02 USD.