My understanding is you cant apply boost to the vacuum modulator. It has to sense engine vacuum to make the shift. Years ago they made mechanical modulators for the T400 for applications like diesels. That worked with a cable hooked to the throttle..kinda like a TV cable.
Someone else may chime in..but 20-30 PSI boost on a vacuum modulator is a bad thing. I dont know the effects of using a check valve since it allows air in a single direction and may not let the modulator see vacuum signal correctly.