l have a '74 Chevy C20 & just rebuilt the trans & now have a shifting problem.l cannot feel the 1-2 shift & 2-3 shift winds way out.Everything is smooth after 3rd.l can downshift & feel each gear, but when l upshift, 1-2 slides in real slow, & 2-3 winds way out & shifts way late.Any suggestions?Doc

l know th400 is use vacuum to shift so l would start there, Make sure the vacuum line is still connected and if it is make sure it is not clogged,

Did you properly adjust the shift cable? Its very important to get the geometry absolutely correct, or else you will need to rebuild that 400 in short order!

the th400 dont have a shift cable first of all so that dont apply here.

there is a way to adjust your shift points. but im not 100% sure on it. l was looking into it like 5 years ago. l believe if you pull your vacuum hose off the transmission and look in the hole theres a little screw in there you can adjust.. but like l said not sure exactly how to go about doing it. google is your friend