I bought a dozen old style Duke 850 a couple years ago. Back before the pan tension adjustment was added. I’ve been wanting to modify them and finally got to it. I pretty much copied how Monte Poole modifies them. I got the pans from him. He threw in the stiffeners.

I used a 3/8” fine thread bolt and Nylok for the pan bolt. The tubing is 1/4” black pipe which is just under 3/8”. I drilled the tubes out to accommodate the 3/8 bolt. The brackets for the spring bolts are 3/4x3/4 angle with a notch cut for the frame to sit in. They are tacked in place.

The triangular connector on the back plate gets the swivel out from under the base to make bedding easier. I decided to try the JC Conners shock springs. I have all my wolf traps set up so I can add an earth anchor or extension chain with a 5/16 quick link. I use 5/16 with all my #5 chain and 1/4” with my #3 extension chains so I can tell at a glance what chain I have.

Right on, looking good.

I like the tubing , I did some on bridgers kinda like that.
I just nuts and not tubing.

The black pipe works great. I also use 1/8” black pipe when I do the dogless mods to my long springs. It’s only a little over 1/4”, .269 ID. So a quarter inch bolt works perfectly.

I've been building some dogless wolf traps and I used the same black tubing and 1/4" bolts on them. I don't understand the sizing on tubing, they call inside diameter of .269 an 1/8" tube and I believe 1/4" tube is just under a half inch ID.

I left the pans floppy on my 850s, I prefer that over their newer pans, and just threaded the ends of the spring pins and put a locking nut on them. I just removed the big wolf swivel from the D ring and replaced it with a crunchproof, which is much smaller and eases bedding, but my dad used welded cold shuts, rather than a swivel there and moved the first swivel down the chain a few inches, to make it nice and flat like you did, to really make bedding easier. That stiffener isn't a bad idea, I've had to straighten a pan or two (and just about every dog) after a catch.