For decades I ran four 8" wheels with 225-50x 15s in front and 275-55x 15" in back, even autocrossing on this combo. The 295s were too wide for proper sidewall support. The 10" were optional for the Euro-only GT-3 street-racer with 285-50x 15" (widest street tire available at the time). Then Gary Hall found out about the 10" wheels and imported about 100 sets which sold well.
I believe Dallara originally designed the Pantera front end for 8" Campys, because the 7" Campys have a poor scrub radius that totally disappears with the 8" wheels. Dallara simply would not have designed such a mismatch on purpose so it likely was a Ford/DeTomaso economic choice to use cheaper 7" at all.