722.3 is good for about 400 NM (except the V8 version), 722.4 were used in models up to 250 NM (280 M104 was the strongest engine delivered with 722.4.

Manual gearboxes of the 124 series are not developed for high torque engines.
300 M103 with 260 NM was the strongest engine delivered with a standard manual transmission.
The Getrag sport gearbox with the 1 st gear „outside the H-scheme“ might be the strongest standard option, as it was built into the 320 M104 with about 300 NM (320 NM? -not sure).