Help! Swapped 302 heads on my 318.

Lifter preload is the amount the lifter plunger goes down in the body at TDC on the base circle of the cam. It's somewhat difficult because the lifter body is usually below the lifter bore at this position but it's the depth the plunger is inside of the lifter body. The most accurate way to do it is put the cylinder your checking to TDC. If the push rod is tight back off the rockers until the plunger returns to top of the lifter body (the pushrod will be loose enough to spin) and install a dial indicator parallel to the lifter body with the contact point on top of the plunger. Set the indicator to zero, and tighten the rocker arms back down and read how much the the plunger goes back in the body.