I am not sure how much power you think you are going to make or how you are going to drive it, but probably the biggest bang for the buck will be a Jerico 4-speed
Jerico RR4 Road Race 4 Speed Top / Bottom Loader Transmission Taylor race engineering in Texas has an adapterplate to the alfa bellhousing as well as solutions for clutch and driveshaft. The flat/fat torque curve of a turbo motor should compensate for the loss of a gear.