If I remember well, after 1988 are better trans. You will need a way to connect your speedo cable: the post '88 use an electric speedo; get the tailshaft housing from an older trans, with a speedo drive, easy to swap. You can also change the speedo gears so your speedo is accurate. If you buy from a junkyard, make sure you get the TV cable and all related brackets, it will help a lot!
This company http://www.transmissioncenter.net/speedometer_calibration_______va.htm
has a lot of stuff!
You need a kit so the converter will lock up, this is necessary for the health of the trans! Bowtie overdrive has that.
TH700R4 is a good choice, giving you a shorter 1st and overdrive, improved driveability! You may have to change your rear end ratio for the best result.
QUite common to modify them, or you can buy an aftermarket already modified.

'57 Chevy Suburban; '70 Chevy Impala.