There are a few things you can do.
-They offer LSD'* and gear swaps but won't offer gear swaps without the LSD.
- if you go to the 4t60-e section on the left it will describe how the gear swaps work
~ if you have a 3.06 fdr(final drive ratio) and replace the diferential with a 3.33 dif, you'll have a 3.33 fdr because you have a 1:1 chain drive but to get to a 3.73 fdr you'll need to change the gears (this is the gear swap engineered offers with a LSD)
And some 4t65 parts will work in your tranny
I noticed a lumina Z34 on cardomain and SSEiJake were using 4t65 trans parts and this is what SSEiJake said when i asked him about his trans:
"My tranny has a 4T65 2nd gear drum w/ modified drum-reaction shell and modified front planet. Input sprang assembally. 3rd roller assembally. Input drum. I think some of the pump parts are from a 65 and I think that it has a modified 65 lockup valve. But the rest is new 60 stuff, like bushings, filter, revs band, modulator, new steels, chain, shift solienoids, pwm solenoids and the gears. It has a 2 clutch OD set up, and 7 clutches on 2nd gear, 12 3rd gear clutches, and 5 forward clutches. I forgot exactly.. but I think the clutches are Raybesto blue plate specials. With a pantom-grip limited slip block, and an intense 3200 TC.
And i found the Raybesto clutches here:
Torque converters from www.zzperformance.com
should bolt right up.
This is the only place i've seen a modified valve body for the 4t60 (but like all of the things i've mentioned so far i have no idea of the quality):
And this page has the cheapest prices for parts that i've been able to find: