I just finished swapping in a Mark IV 454 with Quadrajet into the truck and now I have no speedometer. 700r4 with the NP241 transfer case. Removed the factory ECM and wiring that went with it.
The d.r.a.c. unit is still connected under the radio and the VSS wire inputs into that are fine too. Everything worked with the TBI 350.
I'm at a loss as what could cause this. Any help is super appreciated! I've attached a picture of the drac for wiring colors.

 

Left to right in that connector are as follows:
-empty
-Output to Speedometer
-empty
-Vehicle Speed Sensor Input (from VSS)
-Vehicle Speed to ECM Signal
-Vehicle Speed Signal to RWAL (rear wheel antilock)
-+12V (Ignition)
-Ground
-Vehicle Speed Sensor Input (from VSS)

The +12V ignition switched power source should be powered by the HTR A/C (25A) fuse. I would start by double checking that you have a good ground and +12V with the key-on at the appropriate terminals in the connector.

Here is a wiring schematic from the 1991 full size factory wiring manual I have:

1991 R/V Fullsize Truck DRAC wiring schematic

 

Third wire from the right should have 12V on it. 2nd wire from the right is the ground to use. Start by checking to ensure the ground is good.

With DRAC connector unplugged you should see A/C volts (not DC volts) across green/black and purple/white with transmission output shaft turning by engine. If not (and the wires have been verified to be good), then speed sensor is bad.

If above test is OK - then plug the DRAC back in and turn trans output shaft with the engine. You should have A/C volts across the blue/black and ground wires. If OK (and wire is verified to be good to the Instrument cluster), then the instrument cluster is bad. If no voltage, the the DRAC is bad.

In the center of the connector, the brown wire is to the ECM and white wire is to the ABS. These are only outputs for the DRAC, but you want to make sure they are not shorted.

Hope this helps.