The J79 looks, from the data, to be 2.3 inches greater in diameter than the afterburning Avon and is rather shorter (by more than 2 meters (5.3 vs.
). You'd probably need some cooling scoops, as the J79 does run hotter, but that's not that big a difficulty as the Israelis proved. If memory serves me correctly, the IAPJ article on the J-35 included a side view of a proposed J79-powered version.
Going by the best comparable sets of data I can find online, a RM6C produces 76kN of thrust in burner while a late-model J79 produces 79.3 kN in burner and, if you have the "Combat+" option fitted, 83.7 kN in full burner.