I assume the stock manifolds are cast iron. Am I right?
Cast or pipes-with-flanges, if they are not sitting flat against the heads, you are ripe for faster bolt failures.
If you have access to a large flat surface like a machined table saw top, place the manifold on it. I am sure your local Sears won't mind you using one of their table saws. Just make sure your manifold is clean. You better not see much of a gap in any of the lobes. If you do, you should take it to a metal shop and have the lobes machined flat to each other. You want to torque the bolts using a torque wrench with everything seated perfectly flat to each other for proper torquing and extended longevity. And don't count on the manifold gaskets to fill-in the variances for they will blow out, then burn out.
