Using 8 3/4 with /6

Welding up the spiders will make the back handle like a spool.With 325s in the back, I hated that thing.
With the tire size your slanty needs, it will be fine. Obviously the inside tire will have to slip on every single turn.So try to make as many lefts as rights, or you will be buying tires one atta time,lol.
But like I said I hated that spool out back. One time I shimmed my cone-type a little too tight, and I hated that too.Then I made a tool, so I could measure the tightness BEFORE I installed it.lol. After that I set all my "posi's" the same.But I suppose that might be a lil too tight for skinnier than 295 tires,lol,again. I would have to learn all over.
Bottom line is this; I hated full lock-up in the back, with fat tires.That doesn't mean you will.

Yes your engine will use a bit more fuel around town, cuz,on every turn,you will have to gas it to maintain cornering speed.
On the hiway, it should make as good as zero difference in fuel useage.
But hey, If you put fat enough tires on the back, that the slanty cannot even spin one of them,problem solved,lol! I did that once; N50-15s. Or maybe it was L60s, not sure. But no matter,with a 2800TC, my slanty had no where near enough oats to do little more than chirp just one of those fat-boys.
I don't recall the size codes for Ls or Ns, but they were huge. Ls mightabin 9.25. And Ns mightabin 10.25. These,back in the day, were actual tread-widths, not section-widths like the metrics we now have.If I had to guesstimate, I'd say the 9.25s were 275s and the 10.25s were 295s. They were pretty tall too, requiring a bit of a lift out back,lol, to keep the body off the tires, cuz they were sticking out pretty far.
That's how we rolled in the 70s; I was 22ish. Sheesh, that's over 40 years ago :(