I was running 400rwhp (600bhp+) through my 722.3 gearbox which I had reinforced to handle the extra torque and I still lost the use of 4th gear after only 3000 miles. The power is being sent from a twin turbo M104 stroked with a C36 crankshaft, the same project you're undertaking. So bottom line, don't bother wasting money reinforcing your 722.3, buy a reinforced 722.6 from Southern Hotrod and buy the custom TCU from WhippleM104 on these forums.

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1989 300CE "Project HWA124"
(400rwhp Turbo Technics AMG C36 engine)