Thanks Josh, mine is much rounder than that pictured, and is also much thinner, which could be due to rusting I suppose. Hard for me to see how much use it would be

Very good - thanks gents. I'll need to add some beef to mine as it's very thin

There are two straps - the one that's mounted on the idler yoke arms that are to mount the front tin on, and the one that encircles the front of the recoil spring. As far as I can tell, that strap is to retain the spring in some way, I guess if the bolt breaks.
Anyhoo, I dropped the pivot shaft off at the engineer's place - my crappy plastic calipers weren't good enough to tell me if the OD is 2 15/16 or 3 - does anyone know? My guy is making me up the same type of puller that Toby made for the pivot shaft bushings so I'll extract those when it's made and we should be in position to turn down the shaft to size.
I have a question for the group. The frames really are a mess and I'm contemplating fitting new channel to them - would anyone know what spec of channel they would be beyond the x/y size, such as type of steel? I would get the next size up since I'd need to add 1 to 1 1/2 strap along the bottom anyway to accommodate the smaller diameter D3 rollers.
The other option could be the 4U frames, which are not long and would need to be extended, but that would only work if the dozer mounts fitted in the same relative place

I was looking at that A36 Dan - but wondered if it was a bit light, so thanks for that info. Measurements are the channel base is 5" and the bottom leg is 2.5" and the top leg is 2". According to General Gear, the D3 rollers need to be set off an additional 1" lower while Eddie's instructions gave 1.5". Things I'd need to do are weld 3/4" blocks for the blade mount holes, and additional blocks for the roller mount holes, if I used a 6 or 6.5" base size. But if I stuck with the 5" base, and added a 1" thick strap along the bottom, then I'd already have the thickness required to thread bolt holes for the rollers. Plus I don't know if 6.5 is a standard base size yet.
Would you happen to know any vendor that deals with odd sizes? The difference in width of the legs is one thing - I could keep the top leg at 2.5" and just narrow the plates that span from channel to channel - probably easier. I also think I should get the new bushings and pivot shaft done first, so that I can use them as a reference to fit the channels to the plates and the pivot shaft bushing housing. Channel overall length is about 60"
For the recoil springs, I'd like to fit new bolts. Toby's video stated that the compressed force when they're set up correctly is 5000# which should be easily accommodated by my 12 ton shop press - would you agree? I would like to take the weight, then slice the retaining nut off, and replace the bolt and nut with new. I'm a bit nervous moving them around right now as they're very rusted, but I don't want to sacrifice the springs if I just need to take the compression in the press