No, it won't make any difference, the cam has to much over lap. Change the timing and see how it acts before messing in the distributor, drive it and see if it detonates under light load and heavy load. Keep increasing the timing if the vacuuum improves, but keep driving to check for detonation. You may find no other action is required with a few degrees of timing increase. Also move the vacuum advance to full time vacuum if it's not now, that will help also. You don't want so much timing it get hots or detonates, so you will just have make a change and test it, keep the timing light in the truck along with the wrench required to move the distributor so you make changes as needed. If you find the vacuum increased but it detonates, then limiting the mechanical and/or vacuum advance may be needed, or better grade fuel .
If you don't mind lifter noise, Rhoads Variable lifters will solve the problem, they bleed off at low rpm creating a much smaller cam, thus increasing vacuum. Around 2000 rpm ( depends on valve spring pressure ) they pump up and work as normal lifter do. You get better low rpm torque and vacuum, but still retain the higher rpm preformace. I run them in my 250, it sounds like a mechanical cam with lifter clatter. They also decrease the load on the cam lobe at idle where oil splase is the lowest, so oil choice is't as crucial. My 250 will idle in gear around 19" of vacuum.
The cam over lap and piston ring seal determines amount of vacuum, carburetor size has nothing to do with it. If you scale down the carburetor size enough to increase vacuum, you will be running a very rich mixture.
Carburetor size matters at full rpm.
