Since the light comes on once the car starts to move, then stays on until the engine is shut down I'd be looking at the sensors. Since you did an engine/tranny swap, I'd be looking at the front sensors especially. Could you have possibly buggered one (or both) during the swap?
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.