10 vs 12

Picked up my 1965 chevelle SS.. it has a 10 bolt rearend. are the 12 bolts that much better??

It depends on the application.

If you plan on running it with slicks and launching hard at the track then yes. The 10 bolt it came with can take a lot of abuse if set up right.

My brother-in law blew out his 8.2" ten bolt with a mild small block on street tires. I have been doing some research and I have found that if you have a 10.5", it should be plenty good for the street. The more common 10 bolt in chevelles is the 10.2" which is very weak (so I have found). For help identifying them try going to http://www.oldsmobility.com/10bolt-tech.htm .

For the street it should be the last hing to worry about. The cost to put a 12 bolt or 9 inch ford rear in the car would be about $2500 because finding a 12 bolt 64 0r 65 rear is slim. You need to go custom. Check e-bay if you doubt me. $1500 for new axels, center, gears and bearing kit for the 10 bolt. The cheap way change fluid and put in one of those posi trax units.
http://www.powertrax.com/noslip.html