Why do i need adjustable pushrods?

Thanks for the replies guys,
I actually used a thinner (.27" as opposed to the usual.43") head gasket. My main concern about ordering pushrods to fit, is how exactly do in know where to measure. This might sound like a dumb question, but when setting the preload it can be a real problem if a lifter bleeds down as I then have to guess the setting or risk over adjusting and bottoming out the lifter.