Dang it, one instruction book tells me to use the small o-rings in the front and center diffs, while the supplemental instruction manual says to look at the spec sheet. The spec sheets (I think) tells me not to use the o-rings and different weights for the diffs. Which one is correct?

don't use the small o-rings anywhere, they cause the diffs react inconsistently over longer periods.

only use the o rings on the out drives that seal the diff, not on the internal gears. and upgrade to the carolinas O rings, the stock ones leak.

common diff oil use is 5000 7000 2000 or 5000 7000 1000 front to rear, depends on your style & track conditions. 5, 7, 2 is most common, i'd start there.

so only use the 5mm ones, not the 3mms?

exactly

Any other mods or parts not used that you guys can let me know about? This is my first nitro car, so I'm not too familiar with the install/mods. thanks.

Some people have been saying the stock springs are to short/soft.
I use the O rings in the front and centre diff but not the rear like the instructions said. And run 5,000 in the front 3,000 in the centre and 1,000 in the rear.

It's a full out racing buggy it doesn't need any mods apart from a zip tie on the fuel cap. Although i have seen somewhere that someone put a three brake disks on instead of two.

don't neglect loctite...
loctite is your friend...

i use 2 gaskets on my diffs to help give them a better seal and it gives a better gear mesh too.

Hope this is all right. On the center diff, one of the screws that holds the diff together (one of four on the gear) won't tighten. No matter how much I try, it just spins.

if there's no silicon oil in there put a drop of CA glue in there & put the screw back in. if there's silicon oil in there there isn't much you can unless you buy a new case.

It sounds like youve threaded the diff caseing. You can probably get a new one for not much more than $10.