The solution seems to be the Datsun 5-speed. It's a bolt on upgrade that requires no cutting. Quite a few folks here have done this modification. In his book, Geoff Healey mentioned that there was not room for an overdrive in the tunnel without major modification (they may have used one in a race version, but it would have been extensively modified).
Unless you're after a challenge for it's one sake, the 5-speed seems to be the answer. I'm probably going to hunt one down one of these myself, just to have around should the mood strike me to do the conversion.