TH400 vacuum hookup question

[**grumpy**]

My 66 burb has a 250 inline 6. The PO put a new TH400 behind it when the 3spd manual gave up the ghost.
Thing is he never hooked up vacuum modulator. It runs great but it runs out forever and taxes the motor before each shift.
Being it had a 3spd, there isn't a plugged vaccum port anyway to use, so what's best place to T off to use for the modulator?
I hate winding motor out on every shift. Plus harder on gas.
Thanks
Posted via Mobile Device