I have a 64 chevelle with I believe a 3:36 rear gear non pos...I am wanting to install a 3:73 and a pos unit, I can not find the identification numbers on the rearend anywhere...it is suppose to be on passengers side under bottom near the center. Any other way I can identify which size parts to order, they show several different sizes 7.5, 8.2 and 8.5

You rear "should" be an 8.2 inch ring gear for a 10 bolt. Try these: http://www.chevelles.com/years/64/64rearcodes.html or http://www.chevellestuff.com/1964/rear_axle.htm

Did you look on the top of the axle tube?

The 10 bolt in my '33 has the stamping on the FRONT of the passenger side tube. About 6" out from the 'pumpkin'. (Its a 66 Nova by the way) Used 80 grit to sand off ALL the paint to find it.