Help with a 289 automatic that stalls

[capecodmustang.com]

I have a 67 automatic that stalls when put in gear.

It idles fine but as soon as I drop it into gear it stalls.  I thought it was a vacuum leak.

Could I have a bad modulator on the transmission?

Thoughts?

[CharlesTurner]

Brake booster diaphragm?

[capecodmustang.com]

I'll try it..
Thank you Charles...

[Dan Case]

PM sent