8 3/4 Diff gears and axle end play issue

I installed a set of gears from a 1961 8 3/4 into a 1968 housing. Before installing the gears, I noticed that instead of a round shaft between the spiders, for the axles to butt up to, there is a large square shaft. I didn't think much of it, but now that the axles are back in, I have a bit too much end play on the left axle.
I adjusted the right axle as usual, thumping it to seat, backing off a few notches, etc and the right axle is set perfect. However the left axle has quite a bit of end play. Makes me think that the the big square block in the 1961 gears must not move at all, and isn't transferring the load across to the left axle. This would make sense I suppose since in 1961 the axles relied on shims for end play on both sides.
So now what should I do?
I suppose I could use a right axle on the left so that I have an end play adjuster.
Anyone encounter this before?

I am well experienced in 8 3/4's and am very familiar with their setup

Prior to '64 the open differential had a narrower thrust block (1.25") than the Sure-Grip (1.50") so there were two different length axle shafts. I doubt if you'll be able to make up the 1/4" difference with two bearing adjusters.

You need the correct differential.

That would make sense. I did have to turn the adjuster in a lot more. Probably shoves the thrust block all the way over and still doesn't push against the left axle.
Looks like I need to dig up another center section.

Yup. Took out the Center section and the thrust block is 1 1/4" Wide instead of the 1 1/2" button on the later gears.
I could do a full disassembly of the gears to remove the block and try to retrofit a newer style button, but who knows if it's actually compatible.
I guess I'm looking for another Center section