What pushrods?

I am assuming you have none adjustable rockers. That being the case, this is what I did.

Bolt the head to the block with the gasket in place.
rotate the cam so that the lobe is facing away from the lifter.
place the lifter in it's bore.
place the pushrod down through the head and seat it on the lifter.
start to install the rocker until it meets the push rod and is seated properly on the valve spring.
set up a magnetic base dial indicator so the plunger is over the pushrod end of the rocker and reading about 1/2 an inch.
Finish tightening down the rocker to spec.
read the dial indicator again
the difference in the 2 readings is your lifter pre-load.
You want this to be .015 to .020
You can get away with .030 but I would never go more than that and in a performance engine never more than .020.
When you find your pre-load this is what you do.
Measure the lenght of your current pushrod, subtract the measurement you took and then add .015. That will give you the length of pushrod you need.

Jack