I've sold Procomp for quite a few years and hundreds of sets of their cylinder heads as well and you know the old saying, you get what you pay for. Ya, the Procomp heads are attractively priced and work excellent on a lower compression engine with say a hydraulic roller or flat tappet cam out of the box. With some extensive port work and a GOOD set of valves and springs, titanium retainers and girdles we have used them on a few 496 509 540's with solid rollers in the 700-740 lift range without issues, but only used them due to customer's budgets. They wouldn't be my go to head for those applications....but anyway, if you can budget for the Dart's they definitely by far are superior and will make more power...just my 17 cents...