First I was wondering in someone could identify a rear end serial number?
No. 1235542 other markings A84 and GM 79.

Second I beleive I am running a 4:10 gear and was wondering if it pays to go to 3:73? 3500 rpms at 65 is not good for gas milage.

Third, when the car was running I would get alot of 1 tire spin during
burnouts. Do I need a new posi or would changing the fluid with additive help? The gear lube is quite old.

Any help would be apprecaited.
Thanks

Not much difference between a 3.73 and 4.11. If this is an Auburn posi then throw it away and get an Eaton. If it is an Eaton then you probably need new clutches.

sounds like if you got a posi it is gone you need new carrier.changing oil wont make a difference at all.as for gears i like around 342 ratio good for street or strip also for fun.