Well it looks like you are trying to do the right thing with the carby. 1 carb & moderately sized internals should make the engine run well enough.

What sort of manifold is it? If it is the crossover type where 1 side of the carb feeds two cylinders only, then drawing vacuum from both sides would be a good thing. If it is of conventional design where one carb feeds all four cylinders [like the original did] then only one vacuum source would be needed.

About the only other thing that i can think of is to ensure that the engine is in a top state of tune. The more eficiently it runs, the more vacuum it makes.

I think that the modulator is a sealed unit. You may be able to get it apart, but i don't know about puting it back together.
If you have a spare modulator unit, then give it a go, but don't hack up your good one just yet.