A lip can be good for anti-reversion. If you are staying with stock manifold, I would leave the port size alone and concentrate your efforts on making them smooth and straight. The problem is the firing order and reversion is a problem on most manifold designs. Longer primaries will help but unless you are going to build your own bundle-of-snakes 6-3-1 (1&5 2&6 3&4, each primary 180' out) exhaust, reversion is a concern.
Also, the further you port the stock exhaust manifold primaries, the further you can open up the exhaust port and still have the same degree on anti-reversion.
Jason