This has always been a problem with tri-five Chevys. The only solution used to be using Corvette "rams horn" manifolds and single pipes. You lost the effect of a full length tuned header, but gained the clearance because you could tuck the single pipe up against the crossmember; with the headers, four pipes and a three inch collector just don't fit under there. In todays world, the block hugger "shorties" will get you the same effect - single head pipe to the muff, and you can tuck it up against the crossmsmber and get the clearance you need. The only other choice is a lot of chassis modification.