Do you have the orginial intake/exhaust setup? If not, then measure the boss or flat heights where the clips with bolts hold the two onto the block. There can be a difference in the height between the two. When you bolt/toque the two up one may be loose, the other tight. If you do see a difference in the measurments, use a spacer of the right size and shape to make up the difference. The only other thing, and this should be done first, is to measure the mating surfaces to the block of the two pieces. Have them machined flat if they are not already. Good luck.
