You certainly are going to want to swap the front spindles from the I-6, and upgrade to 5-lug brakes, hubs and wheels. The main reasons to do that include the brake sizing (the 9" 6-cyl brakes were on the hairy edge of adequate even for the 6-cyl car), and the strength of the spindle (V-8 spindles are visibly more robust). In doing that, I believe you'll find that the 6-cyl steering linkage no longer fits the V-8 spindles - different tapered bore in the steering arm, so you'll end up swapping it anyway. Of course, lots of people get away with mild V-8 swaps into 6-cylinder cars and do none of this - it's all about your risk tolerance, I suppose. I'd strongly recommend going the whole nine yards.