Ah of course. Are they proper l34 heads, or are they late vk/stock vl heads? You can tell by the type of exhaust crossover (single bolt or two bolts diagonally across from one another in the middle of each head for the manifold)
The heads, ignition timing (what dizzy are you using - genuine Bosch? Has it been regraphed? How much mechanical advance? Initial?), jetting and mild cam will be the problems as mentioned by gtrboy.
Heads, compression and cam are the most important things. Most stock intakes are alright (especially the pre pollution manifolds) and the stock quadrajet is more than adequate. With that cam a dual plane manifold will be well suited as it won't make power above 5500 or so.