I spent a lot of time on my speedometer cluster when installing the LS1 in my 1988 RCSB. Here is the info on 88-91 speedometers, the cluster has circuits inside that perform the same functions of the DRAC in 92 and later trucks. As in, speedometer driver, speedometer correction, RWAL (antilock brakes), cruise control output, speedometer out to ECM.

88-91 the Yellow and Purple twisted pair wires come off the trans tail VSS and go up the firewall and into large square firewall connector harness near power brake booster. These wires go through connector in firewall and straight into speedometer cluster. The signal the VSS produces is a simple AC square wave signal, the VSS is a magnet reading a toothed ring with 40 teeth, and that 40 pulse signal is what your speedometer needs to work. 88-91 speedometer clusters then condition the VSS raw signal and send it out to everything else, antilock brakes, cruise, original ECM.

If you are using an automatic trans in your swap just jumper off the VSS wires, at trans, at back of speedometer, it doesn't matter, and run them to your new ecm. I don't know why you would though, usually all your new ECM needs to know is if you are moving or stopped. The speedometer output from back of cluster that went to old ECM can be run to new ECM speedometer input pin. Your new ECM is not controlling cruise, antilock brake, or speedometer, as those functions are retained in original cluster if you uncap the yellow and purple wire and run them back up to firewall connector, twist the as they were originally to suppress any noise.