Vehicle Speed Sensor effect on tuning

I did a magnum swap in a friends 1974 D100 short bed truck. We never installed a VSS. And he has been driving it around for 4+ years without issues. Runs great and it's a completely stock harness, ECU, tune, etc. We ended up later finding a Dakota at a salvage yard that had a VSS with a speedo cable hookup to keep the ECU happy and the truck speedo functioning. We never took the time to install it.