I guess it just boils down to preference... both have strong points.
One of the guys in our 'hot rod crowd' had a trans shop for 25 years. I'll work with him but he's the brains (I'll be the monkey in training).
I'll get his opinion too but I'm leaning towards beefing up the 904.
I would say the 904 has less drag because there's less in it. I'm seriously not joking when I say you can train a monkey to rebuild one of these things.