getting no vacuum while cranking is a huge concern. vacuum in that map sensor hole should be readable while cranking. reason could be a giant vac leak (the intake is loose or something)..
it should start regardless of a failed map sensor. all the same, you should actually just plug the map sensor in, apply vaccum to it with a vac pump, and check the value with a scanner. that rules out wiring
coolant temp at -40? you have to fix that too, its important.. the ecm will think it needs a gallon of fuel to start because its so cold. no wonder its full of carbon, huh
-40 is max resistance; you have a broken wire, the sensor is unplugged, or the sensor itself is screwed. test the sensor, if its good, fix the wiring. 4th Gen LT1 F-body Tech Articles
maybe you can tell us a bit about what you did with the wiring during the swap, if you changed anything, etc..
real bitch about these engines is their wiring is very small and fragile (gm made 'em that way to save weight/money) so it's really common to swap an engine wiring harness into a different car and break a wire or two, just from bending/moving it around
luckily testing the entire harness doesnt even take that long (i just did one..)
also you didnt mention what year car this engine is from, would be nice to know..