To answer your questions i see Hughes recommend quite bit here. I got to ask though why roller rockers and not the stock ones? A mild 383 you wouldnt get much benefit from them. Use that money towards other upgrades that give you more gain.
You would be surprised what a set of correct rockers will do for a engine. What I mean by correct is all the rockers are the same ratio, unlike the stock ones which are all over the place. It will smooth out the power and it will run better all over the rpm range.
I had a customer running a 340 with stock rockers with a MP "508" cam. It ran good, but only pulled 4-5 inches of vacuum at idle and would max out at 6500rpm. We sold him a set of the Hughes rockers and he said it smoothed the idle a bit, was pulling 6-7 inches of vacuum and now pulls quite well to 7000rpm.
