Transmission TH350 - TH400

In our 71' Chevy truck 350ci, we replaced the tranny -TH350 (Mechanical) with a newer TH400 (Electronic). The truck shifts late into second when the engine is running high RPM's and it has a hard time shifting in 3rd. The TH400 tranny ran fine before installing. Is there something needed to convert this electronic tranny to run smooth? We also need to change the gear for the speedometer cable, how is this determined?

an electronic th400? as in it has electronic kickdown or something? mine just has a vacuum modulator. did you put a tranny kit in it or anything like that?

The 'electronic TH400' is actually called a 4L80E, has overdrive and requires a computer. If this is the transmission you have, and you don't have that computer, this is what will happen.

All TH400 transmissions have electric kickdown. The later variants of the TH400, officially renamed 3L80, don't have lockup converters or anything, so there should be nothing else electrical aside from the single conductor kickdown line.