i know this is just of topic
i have been going thriugh this process as well.in the end i have gone for a typ 1 irs 4 speed simply for cost and relieabilty.
porsche 901 not strong enough
porsche 915 strong enough but gearing not
typ 1 irs berg 5 speed HOW MUCH ££££
typ 1 irs rebuilt with close gears
just my views and the way iam going good luck which ever way you go
cheers rob