depends on RPMs
the best answer I can give you is to go to http://www.autozone.com. Enter your vehicle info and go to "specifications". You will be able to find the firing order in a diagram there.
firing order is on top of the manifold (if it is stock).
Firing order is 1-3-4-2.
Go to your local auto parts store & ask them for a chiltons or haynes manual on your vehicle.
A lot of the time it is cast on the intake manifold, It is 1-5-3-6-2-4
The firing order diagram for this car is found in its owners manual. It is also found in the maintenance and repair manual book as well.
Click the link.
Here you go. The best one I have ever seen. Copy and paste into your browser. http://www.boxwrench.net/specs/chevy_sb.htm
1-2-3-4-5-6- weird firing order