Rocker shaft spacing help

i have a 383 with eddy RPM heads and comp cams pro magnum rockers. when i try to install the rocker shaft onto the head the spacers between the rocker arms is too thick. i have a set of mechanical rockers with the springs for spacers. can i use these or do i have to measure and buy exact size spacers? what kind clearance do i want between the rocker body's and the spacers? can anyone tell me how to measure for the correct size spacers?