Will this kill my 904 ?
Not sure if this helps or not, but I raced my Barracuda for a year launching it in second gear along with a myriad of other tweaks to slow my 10.5 car down to 12.0 to compete in Sportsman. It worked REALLY well all year until the last regular season race. 2 weeks before the division finals it started slipping really bad (burned up the clutches, stunk to high heaven)...had to have a friend rebuild it for me quickly before the big race.
This was on a 727 trans...and it had probably 8-9 years of racing on it, so it may have just been time for it to go...but I thought I'd throw that out there for food for thought. I really don't know if I was hurting it doing a second gear launch or not.