[rsavage]

Does your timing cover have a tab at 12 o'clock right under the water pump that can barely be seen ? Did you add an aftermarket timing tab on the cover at the 5 o'clock position as small blocks often have or add a different timing cover? It can appear as that the distributor is in correctly and it is on number one and TDC on the lower tab when in fact the TDC should be on the upper tab under the water pump. Balancer crank keyways determine what tab is used.
Happened to me. I put the motor together, didn't see the almost hidden tab and added an aftermarket timing tab. Had that distributor in and out several times 'till I finally figured it out.