68 barracuda suspension

Hotchkis and Firm Feel both sell quality stuff. You might want to consider buying all your stuff from one or the other. It may streamline the tech support process if you have questions down the road.

If by 4.5 x 8 backspace you mean 4.5" of backspace on an 8" wide rim, I think you should be OK. I'd unwrap one rim and measure real carefully before I threw away the boxes and mounted tires on all four.