If one uses a head gasket that is thinner, the distance between the location of the end of the tappet, where the pushrod sits, and the rocker arm, where the pushrod fits on the top, is decreased. Therefore if a cam has a base circle that is smaller than the one that is being changed out, let's say by 0.015 in, and if the head gasket is likewise a drop from .045 in to 0.030 in, the pushrod length would be the same.
If the cams had the same base circle, the pushrod would be 0.015 in "too long".
In actuality most hydraulic tappets have an range of 0.200 in to help account for changes in growth of the cylinders, heads etc as they heat up or cool down. Typically an adjustable pushrod is adjusted to the mid point of the range or down 0.100 in.
I have no idea if this is what the OP is talking about or what considering I know nothing about the Zippers cam in question. He may have meant that since he is taking the heads off to do the gasket, he would reuse the stock pushrods instead of springing for adjustables.