I think this is allowed. I did this on a 5.7 but I believe 77searay197 has done it on a 4.3 with the steps in the thread. One thing to add is you might want to use stainless cap head (Allen wrench style) bolts to attach the risers to the manifolds. You might get a better fit and won't have to worry about getting the wrench in enough to tighten them up.