Early ones strip gears so no use over 300bhp.
Later type and 6 speeds have no issues with diff upgrade if installed correctly.
ÄÄÄÄhhh...300HP with an Integrale style gearbox, only????!
I used such a gearbox in a Coupe for ~10tkm with 565HP, without failure and a 4x4 (much more stress for gear box and clutch than fwd, only).
Only problem is the 3rd gear at older Integrales... this gear was insufficiently welded and some of them broke at high load. The newer Fiat Coupe Integrale style gear boxes don't suffer from this problem - Integrale user even upgrade their gear boxes with 3rd gear from Fiat Coupe...
Big advantage of the old style gear boxes: you get 3 different strenghtening plates and upgrading gears...
Two friends of mine have Integrales with >>500HP and >>650NM... no problems with these mods...
Nex year my coupe will be back on road with this gear box and 4x4 with >650HP...
Then I can tell you, if it will "survive"...
Juergen