This is the straight cut gearbox I am running that I made also...
I had a smooth case 4spd in my 1000Coupe with a worked A15 and never had an issue...
I ran the std 1000 gearbox in my Turbo car for 4 race meetings and never had an issue. The clutch was marginal which I think helped also as it would slip if you tried to flat change it, but if you changed gears quickly but normally it wouldnt slip, bit of a juggling act to drive fast
The radiator was a $100 aly core Camry radiator, I had to chop about 100mm off the length then weld the tanks on, another no budget project...
beattie- be aware that turbo size is very important, for example an SR20 (2ltr) turbo will be to big. If this makes boost on an SR at say 2500rpm onwards, it wont start to work till about 5000rpm, totally useless.
Perhaps the best thing to do is use a single turbo from a twin turbo Subaru, it should be ok for an A12 or A14.