What most people do is called a drain and fill. Drop the pan, replace the filter and put in the same amount of fluid that came out as a starting point. I’m not sure what a standard depth th350 pan holds, but it’s probably around 3.5-5 quarts...tops. How much did you get in?
I’d check your vent tube as recommended. If you’ve overfilled the trans, it’ll want to evacuate the excess fluid out the vent tube or the dipstick.
Then...
Start the truck, shift it back and forth through all the gears. If you aren’t getting any motion without using the throttle, put it in park or neutral with the wheels chocked. With it still running (once it’s warmed up) check the level on the dipstick. If the oil is low, add some if it’ll take it. If it reads normal try putting it into drive again.
Still nothing, then it’s not moving fluid and further inspection will be necessary.