Charlie Davis has a table of the various 901 and 915 gear ratios that were (at some point) available. Many are NLA or rare as hen's teeth now, but Paul Guard (gears at gte dot-net) can generally rustle up or make any of them you really really really need.
On a 901, 2nd gear is part of the mainshaft. To change it, you either get another transmission, or you swap to the 904 mainshaft. Which is big $$.
There's a decent chance that a 901 1st-gear will stand up to a 3.0 launch, as long as you never "dump" the clutch. Z used to do burnouts in his, and it didn't explode for the several years he owned the car after he put the fresh tranny in.
I wouldn't think that the R&P would be a problem with a stock 3.0 motor. I know that it is in a 2.8 L full-on SCCA race motor, though. The pinion gear teeth start galling, then they shear off the shaft. Making for a very interesting paperweight and conversation piece. "Hey Dave--know what this means??" "Uuhhh... No...." "It means I have enough torque!!"
I am told that the R&P mesh is a problem with built 3.0s and 3.2s. There was a discussion about that a year or so ago on the Racing list over on Rennlist.
--DD