First real drive today and it's holding gear too long before changing up. Kick back is excellent.
TH350 trans. Holding 1st to 30mph & 2nd to 50mph.
If I use the manual side of the dual gate it will drop from 2nd to 3rd around 35mph. Still hold 1st too long though.
Sound like a sticking throttle body valve? Car hasn't been driven properly for years and I know my Road Runner did the same thing. Went OK for a few miles then started holding gear. It was the TB valve.
Also checked the Trans fluid & it's way over the mark idling in Park.
Could this have something to do with it holding gear ? Am I safe to have an overfull transmission?

 

Since you just got the car I would start with a fluid and filter change. Another thing to check is your vacuum line going to the transmission. A small vacuum leak can affect how it shifts.

Is the engine revving high when it shifts. Do you know what your rear end gear ratio is 2.56 would take longer to shift then lets say 3.23

 

Not revving high. I believe rear end is 3.23 or lower (previous owner said 3.69 but it doesn't feel that low).
I'll do a fluid change & see how it goes. Do you happen to know the fluid capacity of a 350 trans? No manuals yet.

 

Capacity is 11-13 quarts, but when your doing a filter change you only replace 4 quarts of fluid. That's all the pan holds.

If you do it your self you'll want a large pan to caught the fluid. No drain plug on the transmission pan.