I wholeheartedly agree with 73RS, check the rocker height/valve lash. I had a very similar situation when I put in a new 350 into my Camaro. I must have timed it every way possible. It turned out that I had several of the valves too tight.

I had always read that for hydraulic lifters you should tighten them up until there is no play in the pushrod. I took it way too literally and tightened them until I couldn't even turn the pushrod, then I even tightened it another 3/4 turn. Finally, after failing to get the engine to fire properly, I backed them all off and tightened them untill there was no "vertical" play. The engine fired up right on the first crank.