"bigger is always better"
since i'm guessing that they are stock smog style heads that have been milled down for more compression, I would say that there is no point in going over .450-.470" lift max on this motor cause the heads wont support any higher of a lift. as mentioned tighening up the LSA on the cam will give it a choppier idle, a would recomend a cam in the 220° @ .050 range with around .440-.460" and a 108LSA. matched with some headers and 2.5" exhaust and some 2 chamber mufflers, I recomend the Thrush welded mufflers (i use them on my truck, and i've had a lot of complements). a 600-650 vac secondary carb will do fine or if you can find one and know what you are doing then go for an older 750 Q-jet. He will have a lumpy idle but it wont be crazy. the problem i see here is that even though he will have a lumpy idle he still wont likely have over 250-280hp due to the heads. so my near stock GMC 2500 with the 340hp (factory) 6.0L will still hurt him.
*for a decent performing and sounding motor you will need to change the heads*