The 6SU7 is a direct sub. A 6SN7 *might* work, if your Firestone is NOT a series-filament-string TV set. The 6SN7 draws more current and has less gain.
Any other 6S*7 I wouldn't try.
You might find these pages useful:
http://www.duncanamps.com/tdsl/show.php?des=6SL7GT
http://www.nj7p.org/Tube4.php?tube=6SL7gt