I second that. I always lock the distributor in big cam applications like that. Its amazing how much better they run. With the low compression, and big duration, and moderately tight lobe sep, that thing isn't going to have that much vacc. anyhow. 5 or 6 max. 8 would be maybe at 1400 rpm. Not sure where your idle rpm is set, but that will have an effect on vacc too. If you are trying to idle it at 800 rpm, vs 1000, etc. Guys always seem to try and want to run the idle speed to low on engines like this, and think its their daily driver truck, and it should idle at 650. It won't. 1000 rpm is probably going to be the lowest happy rpm you will be able to run it. My question, why are you worried about vacc? Brakes or something?
back on the original, once you lock the timing, it will also give you WAY more control over the carb. What happens many times on bigger engines, is, you don't have enough idle air, so, you open the throttle blades way open, to get it to idle at a decent rpm, then, you lost all control of the idle circuit. Cranking the timing, will rev the engine up, then, you can back the idle screw and restore the proper transfer slot relationship, and have control again of your idle mix screws. It won't ping or detonate locked out either.
Frank