GTech, I'd check the voltage with the plug connected. A good way to backprobe the wires is with a straight pin. Press the pin into the wire, and touch the pin with the multimeter probe. This prevents too much wire damage.
According to the wiring diagram I'm looking at, the MAP has three wires. The dark green/red is the signal wire. (I think) the purple/white is the 5v reference wire, and (I think) the black/lt blue is the ground.
Check both the purple/white and the black/lt blue for continuity with the engine block, with the key off. The wire with continuity to the block will be the ground. The other will be the 5v reference wire, which you can check for 5v with the key on.
These wires make connections to several other sensors. If you don't get a 5v reading on the MAP wire, check the plug-ins for the TPS, ECT, IAT, and VSS; they are all connected. Since this was an engine swap, I think the problem is a poor connection somewhere.....
Luck,
ROE