Sorry for the delay in getting back to you on this! But I agree, the math on that looks right for the rears with the info from your current set up. And with a 1/2" spring offset you'll have plenty of room for 275's. 285's should actually fit too, as I mentioned I run 295/35/18/s on my Duster on 18x10's but I also had to trim the quarter lip about a 1/2" to pull that off at the ride height I'm at, in addition to the 1/2" spring offset.
There are actually like 6 different widths for B-body 8 3/4's, 4 that are the non-tapered axle type. A '67 is narrower than a 68-70 rear. But like I said with the info on the fit from your current set up I think your wheels will work great. You can take a look at the rear end widths here
An accurate 8 3/4" rear axle width list
Yup, in the front 275's are possible without any radical modifications if the backspace is right with a set of 18" or larger wheels. The 18" wheels clearing the outer tie rod ends allows enough backspace for it to be possible. Spacers aren't necessary if you get the backspace right, I'm running without the 3mm spacers now. It was just an experiment as my wheels touch the frame at full lock, but that turned out not to be a backspace issue but rather the steering stops. Or lack thereof actually on a set of QA1 LCA's. Thin spacers are no big deal anyway as long as they fit well and you have long enough studs to still get proper thread engagement.
When you machine the hub bores just remember that you'll probably have to do something custom for a wheel cap unless you machine them to a size for an existing wheel cap (which wouldn't be a bad idea). In the front I used a pull though cap that was the right diameter, cut the end off and epoxied the wheel cap that I'm using in the back.
Here's some pictures showing the wheel cap, and how big the 275's are up front
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