I don't think your going to say any gain going from manifolds to a shorty header IMO and my experience putting a set on. You will gain some with full length headers, but if this is going to be a daily/cruiser, lower RPM, manifolds will work just as well. Magnum ones flow a little better, but you need to do something with the kickdown linkage---at least I did (I put pacesetter shorties in place of stock magnum manifolds.)