Around what speed does your TH350 shift from 1st to 2nd gears and from 2nd to 3rd gears when casually cruising?
I have had my 69 cutlass for about 5 years now, it is a work in progress driver.
The motor and Trans are not original but was told they came out of a 72 cutlass. Since I bought it the transmission never really shifted right. I have been manually shifting the transmission through low gears when I drive it and the transmission seemed to work fine. I finally decided to look into the issue and through research on here found that it was most likely a bad modulator. bought the new modulator and went to put it in, turns out the old one was never hooked up to the vacuum line.
So I put in the new modulator since I already bought it, ran the vacuum from the manifold and the transmission shifts on its own now.
my concern is that the new modulator needs adjusted as I feel it is up shifting too soon, 1st to 2nd at around 20mph and 2nd to 3rd at around 30mph.
Does this seem right?

 

Shift points will vary depending upon the rear gear ratio. Lower gears will rev quicker and therefore shift at a lower MPH.

If you think yours is shifting too soon, then adjust the modulator to have it shift at a point that feels right to you.

 

The modulator will change your shift point maybe 5mph. Be careful with the adjustment because you can turn it in too far and the screw will go into the modulator body and your adjustment will go away.

 

A 2.73 rear I have has the car shift under normal acceleration at @15 and @30

 

Thank you all for the replies. Driving the car yesterday it sounds like there is still a vacuum leak somewhere so I will fix this issue first and it may help. On a side note the car starts, idles and runs so much better with out the massive vacuum leak from the modulator being disconnected.

 

Don't recall the exact shift points but, yes the current shift points posted seem very close to what I remember with your 1-2 slightly late IIRC but I'd prefer it that way.
Good luck!!!