hmm...... how to explain this.....
lol
well you could mock assemble your engine, then install cam and two lifters on a cylinder. put a dab of assembly grease on lifter to protect cam.
get a light spring from the hardware store to replace the valve springs in the head. its just there to apply light tension to the valve and the retainer and locks. dont forget to install your valve seal!
put the head on the block, with a gasket - and snug it down. assemble the pushrods and rockers on the two lifters you installed. add a little oil to the parts just to keep action smooth.
now roll the engine over, until the cam has come to full lift on the intake.
now the rocker will have rolled, and the valve should be open.
looking threw the light spring from the hardware store.... get a feeler gauge and measure between the valve retainer and the top of the valve guide seal.
that is your retainer to Guide clearance.
at the very least, it should be .060
.065 makes me feel better though.
and do the same for the exhaust side as well.
from what I remember in the buick powersource manual, any cam over .420 lift will have an issue if your using PC type valve seals.
I hope this helps, please tell me if your totally confused.

A.j.