You could look into the 97-99 Corvette stock LS manifolds. I am putting a 5.3 into a 47 Chevy and found the same clearance issue. I have ordered a set of the vette units but have not received them yet, so the jury is still out. Seems to be the main units to use in this case for clearance. If you chose the vette units I recommend going to a Corvette forum and contacting the vette owners. The first upgrade they do is pull the stock exhaust and replace. A much cheaper price from them than auction sites, saved me a good amount.