TH350 shift point question

[MagmaJct]

I have a 1971 C10 1/2 pickup. As far as I know it's all stock except the carburetor and larger tires.

Problem/Question: When accelerating from a stop, or when accelerating out of a turn, I ease into the accelerator. When you only get 10 MPG you do all you can. As the I ease into the accelerator the transmission with up shift (into 2nd?) too soon and the rate of acceleration decreases.

As I continue to ease into the accelerator, the transmission will then downshift again continuing the acceleration I had before. It has been an annoyance for some time, but now I'm concerned this will lead to excessive wear and tear on the transmission.

Details: As far as I know stock 350. Tire size 235/75 The carburetor is a Carter AFB equiv. to the Edelbrock 1405 with the add on electric choke. Engine vacuum is a little low, but I can't find a vacuum leak and engine compression is in spec according to my Chilton's.

Ideas?