I'm pretty sure that the TH350 wants manifold vacuum, you could easily tee into one of the lines for more vacuum points. HRL should be able to enlighten us on the tranny modulator but I think I remember them using manifold vacuum.

1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy