If the purpose of this excersize is to check valve to piston clearance, the height of the pushrod cup in the lifter isn't critical, as long as it's basically solid.

To determine pushrod length, I would just pump one of the lifters up with oil and use that with the checking pushrod, cam on base circle, adjust length to just zero out the clearance.
Measure the length of the checking pushrod, add to that dimension the amount of lifter preload you want.......... then order that total length.