Call Manton ( www.mantonpushrods.com
) and order one intake and one exhaust adjustable checking pushrod, (real inexpensive).
Completely assemble the engine, heads, rocker arm assemblies. Install the checking push rods, adjust, and roll the engine over several times to make sure they are the correct length, check that the valve moves its full range. adjust again if necessary roll the engine over and over to check.
Continue to do this until full lift figures, from the cam spec sheet, are duplicated measured at the valve stem and corresponding ratio number measured on the lifter body (not the cup).
Remove the two adjusted push rods and send them into Manton, a couple af days later you will have the correct length push rods.