If the TPS is connected and he's getting these numbers, I'm agreeing, there's got to be a wiring or other connection problem. A poor or nonexistant ground connection could be the culprit as well, so let's check those ground connections! Make sure there's less than 0.5 ohm between the battery ground and the EEC's chassis, and even then be sure the connection is sound.
Whether it's an internal (EEC) or external (wiring) problem, it's going to be some good ol' harness diving and tracing.....not fun, but necessary. Since you mentioned it happened all of a sudden, I'm thinking that you have a pinched wire somewhere, as happens in so many transplants.
I just don't see how you're getting almost 5V on the TP's signal line (going back to the EEC) if the TPS is connected and there aren't any shorts in the system....usually this happens if the TPS is shorted internally. If it's an identical voltage to the +5V line, I'd say that's strong evidence that you indeed do have a short across lines somewhere, somehow. Do you have a spare computer you could try temporarily, just to rule out a bad computer?
With the TPS connected, you should have ~5V+ on the Vcc (or whatever you wanna call it
it's just a regulated voltage rail from the EEC) line, 0V on the ground line, and 0.7V < TP < 0.95V for the remaining line.