bit of an unusual one
I have seen 75s with Cosworth engines from a Sierra and BMW engines in
the biggest obstacle to big power will be the rubber do-nuts in the propshaft, too much torque and they let go, power however is a different matter, they will take 300bhp
A porsche 944 has a similar arrangement of the gearbox at the rear, an Alfa 6 has the 2.5 V6 and a conventonial gearbox but are highly sought after and most have long since rotted away
various supercharger kits are available from the US to increase the output or the 3.0 24v from the GTV or 166 can be fitted once adapted from FWD with relative ease.
it is slightly off-ish to recommend other sites, but hey we are all alfisti, alfaromeo75.com, alfabb and gtv6 froum in particular may meet the needs of the specialist especially the gtv6 site where they are more hardcore.