50 flatty idles well revs okay, but will not drive down the road.

[twc]

I have a rebuilt 50 flatty with a 97 holley/ford 3 spd tranny. all new and rebuilt, will idle well and sounds good except for a bit of a stutter when you hit the throttle. I get it on the road and it cant handle the load and coughs and stutters and dies. it will always start back up but not quickly. dont know if it is timing or carburation. i have an electric fuel pump from napa, and an in line regulator on full flow (5). any ideas why it will not get going down the road?