I ran into the same exhaust problem putting in a 350 SBC into my D50 4x4. I put a body lift on it and used block hugger headers. Your problem is no one makes a smog legal block hugger headers that I could find. Is the 5.3 longer than the 350. Reason I ask is I was able to install a large aluminum radiator and mechanical fan with shroud without cutting away the radiator support like you did.