If Pip can fit a supra gearbox for you than I can guarantee that you would have a bullet proof gearbox. Supra\'s gearboxes are capable of up to 1600 RWHP.
However, I do know that Pip has fitted many T56 magnums, so he might suggest that for you.
Frank, I learnt one majore principle of life and that is, if you doing something, do it once and do it right. In my honest opinion, it just does not make sense that you know in your head that something needs to be done and you leave it for the other day.
The sooner the better. Mate, go manual and get it over and done with.
Take my example, I rebuild the engine, then send the car away for ECU, now I have to send it for the wastegate and mapping. On the other hand, all these jobs could have been concluded on the very first visit the car made to WGT, and I could have saved arround a 1000 pounds in RECOVERIES
I hope the above make sense