i did what brian did when i swapped in my 350. i tied into the factory harness for ignition,alt, starter, a/c etc. i started with a 22re so i took all of the unused ecu wires and cut them off just a few inches from the ecu and capped them with shrink tube. for instruments i used the toyota instruments and used metric to sae adapters so they could screw into the chevy block. to resuse your tach (i am going from memory so double check me) connect in on the (-) side of the ignition wiring. to calibrate i temp wired in a 0-500 ohm potentiometer in series but i forget what wire, then dialed in the tach with a temporary tach that you would use to set timing. then measured the resistance of the pot and installed a resistor for that value. the tach was not very linear its accuracy fell off on the higher end of the range, but not too bad.
hope this helps.