You would need to drain the rear axle oil before, or as you removed the cover. I don't think I would bother just yet, as there is an easier way.
First count the teeth on the extra ring and pinion you have, then divide and let us know what ratio that is. Do they look used? if they look used, and turn out to be the ratio you should have had stock, then it's a safe guess that the faster 3.55 gears are installed already.
To find the ratio, without removing the cover, I would jack up one rear wheel, put the transmission in high, (1:1), pull the spark plugs so the engine turns easier.
Then have someone turn the engine over by hand as they count revolutions of the crankshaft. At the same time you count the revolutions of the rear wheel, and when it has turned 20 times stop, and ask the guy turning the engine how many turns that was.
now for figuring purposes lets say he turned it 35.5 times, divide that by 10 and you get the ratio of 3.55.
Now don't get confused by the 20 turns of the wheel and then only dividing by 10. The reason is the other wheel was still, and the differential action makes the one wheel that does turn turn twice as fast as it should. It would be less confusing mathematically to jack up both wheels and let them turn together, but practically it's hard to keep them turning together, another person would be helpful if you did.
You need to turn the engine with the transmission in high because you have an enclosed driveshaft and can't see it turn.
For those of you that have an open driveshaft just leave the transmission in neutral and turn the drive shaft instead of the engine.
Confusing enough?
Grigg
