I don't think you'll pick up 30hp across the board but I do think you are generally correct. And I bet you do pick up nearly that in at least part of the power range. Most old school manifolds are pretty awful at power delivery.
Right I didn't word that great, more like 20-30 lb-ft across the board and 30hp PEAK gain. It seems to run out of breath somewhat around 5800 RPM. I have the shorty headers currently and not only are those mediocre at best themselves but I feel like the tiny 2 1/4" head pipes that snake super tightly up over the steering linkage then back down are creating a good bit of restriction. Engine is a stock 5.9L Magnum short block with custom-grind Racer Brown hyd roller cam around 222° @ .050" intake and 224° exhaust, 110 LSA, IIRC. And has heavily ported Edelbrock heads with port-matched RPM intake and 750cfm Street Demon carb. I think it *should* pull to at least 6k with the whole combo matched correctly.