What sort of positract would a 1970 Cutlass "S" with the 455 engine need? Especially if it is going to be used for drag racing.

 

Drag racers use lower gearing like 3.73, 3.91 or 4.11
The motor runs at higher RPMs with lower gears.
The higher the rpm, the more Hp it's pushing, blah blah etc etc.

Bottom line.... they only have 1/4 of a mile to squeeze as much juice as they can out of the lemon.
It's easier to get there if you hit the ground running instead of walking.

 

Eaton posi with some Richmond gears (3.73 or 3.91 or 4.11). If you plan on just using it as a drag car go with the 4.11

3.73 will give you a little more streetability out of the vehicle.

 

Go for a 3:42 gear and if you need more put smaller diameter tires or slick on it like a 25" and that will give you an effective ratio of about 3:70 or so and when you drive it on the street use the standard 26.5" tire for the true 3:42 ratio

 

Is there a certain brand I should get? Are there brands I should not get?

 

Go here and contact Jim.
Most helpful rear end builder on the Olds/Buick forums.
http://www.jdrace.com/about.htm