500HP from a 350 with double hump heads seams a bit high. From what I`ve heard , it would be quite a stretch to get ANY stock cast iron heads to flow enough cfm to achive 500hp.(double hump or not) I`ve got dartII sportsman heads on a 383 w/ .525 lift cam that only runs about 450hp.
For the price though, even if the #`s are exagerated,it still sounds like a reasonable deal. Especially with the installation help.
MY .02