I'm quite sure it would be easy to find an 8HP90. When I break the one I have, I'll certainly pay more attention on where to get one.
Will that be cheaper than a more stout rebuilt stock trans though? He might never break his stock transmission so this is all assumption.
I think (my gut feeling with zero data to back it up) that the stock trans will be fine if it isn't subjected to the shocks of drag racing and if the torque management is left in place during the tuning process. I could be wrong and if that's the case I'll have to eat the cost of fixing or replacing the 8HP70. I'll cross that bridge when I get to it. The same could be said for going with a conservative 6-7psi on boost and leaving the engine internals alone. Some people do that and don't suffer any consequences other than the additional power (and lightening of the wallet). Other folks haven't been so lucky. For me, the degree to which things get over-engineered is going to be primarily driven by how much free time I have in the coming months. Right now, I don't have any. Cheers!
im pretty sure the HP90 can handle 600-700 whp hellcats are in the 620ish whp now with some close to 900+whp and I think the demon has the hp90 also