x70s have a double-layered fender lip in the back, it's actually a PITA to roll Plus the AWD models have a 10mm wider rear track than FWD cars do, so our stock wheels are ever so slightly less tucked in from the factory
Mike, wagon looks awesome dude!
I don't remember if your comets are 7" or 7.5" wide. For arguments sake, if you used a 15mm spacer on 7.5" wheels, they'd sit 0.4mm further in than my wheels. With a 20mm spacer on 7" wheels, they'd sit 1.7mm further in than my wheels do.
Reference pics of the wheels on my wagon:
Wheels are 17x8 et34 with 235/45/17. I get some minor rubbing but only when I have a few hundred pounds of stuff in the trunk.