Just purchased the above car; 2 owners since new, genuine 94,000 mile on the clock. Will be totally rebuilding the car, ie: roof chop, mini tub, 2-door conversion, etc... want to start with the drive train. Looking for a 2V head for the engine and want to know what would be the ideal gearbox and diff for the old girl. Had a friend 20 years ago who bolted on a GT Toploader to his mildly worked 250 2V, but heard that the gearbox tunnel needed some modification. I understand that the brake booster needs to shortened to fit the 2V into the engine bay, more concerned at the moment with getting the 2V head and which gearbox to use.

Hows it bro, didn't the cortina came with the 2V as well? I know the later model cortina came with the 250 crossflow block so you should check at the wreakers for the booster. not 100% sure.
you could get away with a 4 or 5 speed supra gearbox if you want. theres lots of conversion kits on the market or check your local area for a conversion specialist.