[jocko]

6 lug steelies (original wheels) are not disk brake compatible - the taper on the inside face is canted and interferes with the caliper. It's not necessarily a backspacing issue.

Are you using truck or car rallies? (i.e. are you retaining 6 lug or converting/converted to 5, etc). Reason I ask - I can't confirm for you that old vette rallies will clear, but all truck rallies will clear (stock size disk + caliper combo). But, truck rallies are 6 lug, car rallies are 5 lug.

You mention your old rallies - they are definitely newer than your truck, and rallies do fit with disk brakes, assuming you're going with a stock-size rotor, etc. BIG brake kits will not fill rallies. But, if you're buying a kit for stock size application, rallies will be fine.