im planning to build a manifold because my stock turbo II manifold is getting cracked and i would like to know how how big i can use for primary tubing? is so i can use the stock turbo and i was thinking using 2" like RB have in the headers but i would like another opinion. thanks

I'm not too sure on the diameter, but I know you want them kind of small the the exhaust gas travels at a higher velocity.

well how small i have to get the tubes?

When you look at your stock manifold...the oblong ports are no bigger then 2"...if you are going with a round tube, i'd say something more like 1.5" would be better...the headers are larger to allow a good free flowing path for the exhaust, but you want to keep rapid-hot pulses for a turbo...get what i'm saying?