It's pretty easy to determine if the pushrods are too long/short; with the cylinder at TDC on the compression stroke observe the clearance between the lifter top disc and its retainer ring.

If the disc rests up against the retainer ring the pushrod is way too short, if it sits more than .080" below the retainer it's too long.

If the pushrod length is OK and the lifters won't pump up you might have a set of small block lifters.