I have a Jatco 3speed in my RB20 powered Valiant Ute.
Has a 5000RPM stall speed converter, TransBrake, high flow fluid mods, and all the extra clutches. Good for 600BHP.
If I wanted to stengthen the box at all I'd need to get a Titanium input shaft made up ($500 bucks odd).
They are identical to a Chrysler Torqueflight/Ford C4, but the machine work and material quality is far better (Jap built).
For 900 HP, it will work, it's small and light and has 3 gears.