Since SBC pushrods are the same length for both intake and exhaust, you only need one checking pushrod. Buy the one in the range of a stock length pushrod for that engine.
All stock SBC pushrods are 5/16" diameter. Providing you use the proper guideplates, it's better to upgrade to the stronger 3/8" diameter ones.
SBC with OE hydraulic roller cam is 7.195" long
SBC with retrofitted hydraulic roller that was originally a flat tappet is 7.290"
SBC with normal flat tappet lifters is 7.794"
Comp cams checking pushrod #7702-1 covers lengths from 6.800" to 7.800" which should cover the length you need.