milage wont matter, IIRC correctly ron had rebuild done on his(stock rebuild i think). and saturday is right about the input and output shafts with slicks. sub 1.9-1.8 60's are very harsh. thats why most run drag radials on stockers. if you build your trans now it will be cheapier then when it fully snaps and your towing it everywhere . IMHO i dont think you need full race tranny(you dont visit the track that often and dont drive crazy far), you dont beat on your car for the most part. i think just upgrading a few of the parts as you next mod would be smart
you have gone this far, why stop know. hell think about how much you will have to pay to demod that thing(400 cam removal just to name one). you never get your money back out of these things for the most part. most parts you will lose 20-50% on.
think insurance on a GP vs brand new most powerful ever TA(even over 25 that will be some change)you have a custom car enjoy it
tho many feel a 400 hp daily driven GP is to much upkeep. so in your build up dont forget to include another car to get around or some nice friends to drive everywhere