904 internals

At that power level you need really good parts and blueprinted specs and since you say it's a 75% race car I'd go with a manual valve body. I haven't used a Trans-go TF-3 kit so no idea how well it works. If you go with a aftermarket valve body most have a recommended front band lever ratio, use what they say. I suggest tightening up the clutch clearances from the factory specs. I'd shoot for .040 on the rear clutch and .055~.065 on the front clutch. The clutch pack clearance is set by selective thickness snap rings. You also need a multi spring pack for the front clutch to replace single wound factory spring that likes to let go at high power/rpm levels. You also need to tighten up the total end play. Factory specs were extremely loose. Get it as close to .015 as you can so the gear train isn't slapping back and forth. Install good gears in the front pump and blueprint it.

904's came in several different varieties with several changes over the years. Some only used 3 clutch discs and 3 pin planetaries. Others had up to 5 disc clutch packs. At your power level you need 4 pin planetaries and preferably 5 disc clutch packs. You'll find those parts in the 998 and 999 versions (look identical to 904's but have the beefier internals). If you don't have those parts www.wittrans.com sells them reasonable.

I'd also suggest a torque converter with a billet snout as their prone to breaking at higher power levels.

I keep hearing the better 904's are the 72 and up versions but I'm much more versed with 727's so don't quote me on that. Another thing I'm not sure of is how much power the stock input shaft will handle. Maybe someone else can say if it's up to snuff. Hopefully Tony "70aarcuda" will chime in as he's done a lot of 904's