[Jack21]

Have similar setup. Try hooking vacuum line to manifold vacuum port on carb for testing. Timed vacuum port isn't giving enough vacuum to work vacuum advance until engine is under load at part throttle (cruise). The return spring in the vacuum advance is not too strong to work properly. (Keep your fingers out of the way when connecting vacuum line, and plunger pulls in!) Is your distributor plate binding? It should rotate freely with vac unit out of the dist.

Gave up on the Duraspark, and got the MSD dist w/ vacuum. It too doesn't come with an adjustable vacuum, you have to buy one from Crane or Accel. And...the MSD dist takes a '57 - '73 Delco (Chevy) vacuum unit. Nice unit though. Basically a HD, ball bearing, Delco dist on a Ford stalk. Roller cam - steel gear. Flat cam - iron gear.

The 351W tends to like 12 - 14 deg initial, 20 deg centrifugal @ 2500 RPM, but only 8 - 12 deg vacuum. Any more vacuum and it will develop a miss at light load highway cruise. (Which is why the OEM non-adjustable MSD vacuum was replaced.)