Need help finding idle speed on my 1950 chevy automatic powerglide

hi.
155 horsepower 1968 250 CID I6 Engine
This engine was capable of producing 155 horsepower at 4,200 rpm and a torque of 235 pound-feet at 1,600 rpm. It came standard with a three-speed manual transmission, while Powerglide and the Hydra-matic transmissions were available as optional upgrades.
or try this link.
Chevrolet Wikipedia
https://www.classicmuscle.com/resources/chevy-ck-trucks-history

Unless the problem is in the linkage for the shifter you will need to tear down the transmission to find the problem.

(440) Automatic Transmission Input Shaft Speed (A/T ISS) Sensor Assembly

You want a 700R4 from the 1980's -4 speed
from a BPO car


Convert the carb to throttle body

Do this

This problem is commonly caused by a dirty automatic idle speed control valve and throttle valve but always run a OBD2 fault code as well as the cleaning procedure. Buy a can of throttle valve cleaner (do not use carburetor spray cleaner!) from NAPA or Carquest (made by CRC chemicals) and spray it into the air intake while the engine is running, use up about 1/2 the can, engine will try to stall hold the speed up, shut it down and let it soak for 30 minutes, restart and blow out the remaining fluid, shut it down and disconnect the negative battery cable for 5 Min's to reset the base idle control. If the above cleaning doesn't cure your issues you need to have the computer controlled automatic idle speed control system diagnosed and repaired by a dealer or electronic engine control repair shop.

If it is an automatic trans, the idle air control valve should allow extra air into the intake to bump up the idle speed in drive.
If that is not happening, you could have dirt in the throttle body, or a faulty control valve.

Check the throttle position sensor, or the idle air control valve

Are you sure you have a powerglide i dont remember ever seeing one on a powerglide those trans didnt have one from the good ol days just a throttle linkege and modulater valve with a vacum hose to the intake mannifold.

Thought you had an old "slip & slide powerglide" ! I like the way they originally cursed them, then all the racers wanted them all of a sudden. Anyway, if you have the TH350, which your obviously cruising and shifting manually, just go with gear selection that sounds good to you, RPM wise. I never look at a tachometer, all by ear. The TH 350 are pretty strong too. Then of course there's always a shift kit that will make it bang into next gear & chirp tires for cruising too.

You just need to Locate one from a 350 Chevy. They have the Same Bolt Pattern.