I assume you are aware there are adjustable modulators for the TH350 and TH400?
I've not had to adjust one but they are easy to remove and maybe there is a way to bench test yours using a hand vacuum pump with a gauge.
Also you could check to see what vacuum you are getting at the hose end that connects to your vacuum modulator.
I think it is the ones with a red stripe that are adjustable Like I say I've never done one but they are fairly simple in operation
Here is something I just found not looking too hard
http://forum.73-87chevytrucks.com/sm...?topic=25669.0
And here is something from the TCI site: (a new one is only 16 bucks)...
Adjustable Vacuum Modulators
The vacuum modulator is a vital component of automatic transmissions. It tells the transmission what kind of load is being put on it, allowing the transmission to react with the proper line pressures and shift points. Over time, modulators can develop leaks, suffer ruptured diaphragms, become bent, etc. Not only can this cause annoying drivability issues, it can lead to premature transmission failure.
TCIŽ comes to the rescue with this line of adjustable modulators. Renew performance, and gain the ability to slightly raise/lower part throttle shift points and line pressures. Simply adjust the screw inside the vacuum nipple for more or less diaphragm pre-load.
Allows you to fine tune your shift points and shift feel.