The 722.6 is probably the only official 5 sp to hold a big motor.

But, think about this, if your car isn't likely to see an additional 100,000 miles in the near future and you consider the trans to be a wear item, a 722.5 would probably work. Some sort of shift kit / higher line pressure would probably keep it going.

The 722.6 needs a electronic controler, take a look at the performance section on this site, someone is developing a stand alone control for just under $ 1,000.

The 722.3 and 722.5 are basically the same trans, the 722.5 has overdrive in the tail of the case. The two trans even take the same pan gasket.

The 722.4 is the small trans

I'm only investigating this because I have a ragged 91 300SL that is turning into a track day car. The end plan is to use a 97 M104 / 722.6 but I'm real tempted to fix / replace the currently bad 722.5 and run it until the 300 - 24V motor expires. Using a 722.3 would give me a wider range of parts.

From this thread
722.4 and 722.3, interchangeable?

Torque capacity
The designation W4A0xx tells us the maximum input torque in kpm (kilopondmeter).
1kpm=7.23lb/ft.

722.0 (W3A040)- 290lb/ft
722.1 (W4B020 or W4B025)- 145/180lb/ft
722.2 (W4B025)- 180lb/ft
722.3 (W4A040)- 290lb/ft
722.4 (W4A020)- 145lb/ft
722.5 (W5A030)- 216lb/ft
722.6 (W5A580)- 578lb/ft

G-models
720.1?x (W4A018)- 130lb/ft
722.39x (W4A028)- 202lb/ft
722.61x/63x (W5A330)- 216lb/ft (Used W5A580 in the larger engine models)

Note: The 300D/TD/CD/SD from October 1984-on (with the 2.88 rear end) used the weaker 722.4 transmission.

that from superturbodiesel.com.