How low is your vacuum before starting to consider changing the modulator to a low vacuum version vs stock?
Going on a 1969 TH400

 

I would guess when the vacuum is low enough to prevent it shifting correctly.

I have built some 400s that went behind some pretty rowdy with bottom of the page cam grinds, and never needed a fancy modulator. How much vacuum do you have and what problems are you having?

 

If you have enough vacuum to run power brakes I think you should be fine without the low vacuum modulator.