Check the dizzy advance. Unplug the dizzy vac line and plug it, and using a timing light, check the timing at idle, and at the following rpms:
Federal
Governor (crank deg/rpm)
Initial 0/l600
Middle 9/2800
Final 19/6000
Vacuum (crank deg/mmHg/inHg)
Initial 0/ 80/ 3.15
Middle 12/150/ 5.90
Final 23/280/11.02
California
Governor (crank deghpm)
initial 0/l600
Middle 9/2800
Final 19/6000
Vacuum (crank deg/mmHg/inHg)
Initial 0/ 80/ 3.15
Middle 12/150/ 5.90
Final 20/360/14.17

Use a hand vacuum pump to check the vac advance, but I don't think it's vac.

Does it seem to fall off in acceleration as you near the upper limit speed? Any jerkiness at the upper limit? Will it wind up to say 5500rpms in neutral? Did you ever connect the return fuel line? Is the choke open, and staying open?