i need help w/duster engine

Melissa,
the only way to get an accurrate distance is to do what's called claying the piston. You remove the head and take regular modeling clay and flatten some out so it is about 1/8" thick and goes accross the valve pockets and a little of the dome (flat surface). Then you remove the heavy valve springs from that cylinder's head and replace with light weight checking springs. The replace the head. rotate the engine slowly by hand and the valves will open and close with the cam as if the engine were running, making marks in the clay. After both valves have openned and closed, pull the head and measure the clay thickness and THAT is the true piston to valve clearance. I agree with the above posts. A roller cam is NOT a drop in regardless of what you read or want. It needs to be carefully done or you'll need a completely new engine when the parts come apart. Also, the new member here OU812 has the best drop in lifter for small block mopars.