Suspension/Chassis Suggestions - 70 Dart Swinger
I'll probably stick with 4 matching wheels and go with the 245 or 255 width tires, will def need to measure it all out once I get it assembled.
That's a pretty good plan, there's more tire options especially in 245. That also opens up an 18x8 or 18x8.5 as an option, although you could still run 18x9's and then go to a wider tire later if you wanted without needing to change the wheels again. 255's have a decent selection as well and are a little taller in the sizes you'd be close to running, they still fit on an 18x8.5 but an 18x9 would be better.
Personally I'd still run the 1/2" spring offset, you definitely don't need it for 245's but it would be easier to get 255's in there without having to be absolutely perfect on your backspace.
Sounds like you already have an 8 3/4, not sure if it's for an A-body. With a square set up it would be easier to get a 65-67 B body rear axle. The "problem" with A-bodies is that the track width is not the same front and rear with an A-body 8 3/4 even with BBP axles. So for an 18x8 front wheel you need somewhere in the ballpark of 20 -35 mm of offset, and like 25-35 for an 18x9 (tire dependent of course). But in the back, with an A-body rear, you need more like 12-15mm off offset for those same wheels. That gets a little better with the 1/2" offset and you can do something like run +25's all the way around.
Now you can run 18x8's with a 245 with like a +20 offset front and rear, it will clear up front and in the back you can add a thin spacer if you're running drums or maybe just get away with it if you convert to disks (which widens the track a bit). If you have the 1/2 offset it will just work.
But with the wider 65-67 B rear you can basically run the same offset front and rear, it solves the front/rear track width issue. And that opens up a lot more wheel choices, finding the same wheel in the same width but with different offsets reduces your options, and of course it means you can't rotate wheels either even though you run the same tire size. On my Duster, with the wider 68-70 B rear I'm pretty close to even, running a +35 in the front and a +38 in the back. I have a wheel stagger but I could run 18x10's all the way around with a +30 to +38 offset. With the Dart wheel wells being a bit narrower the 65-67 B works out a little better.