If you bought a GM crate motor and it came with the matching GM Harness, ECU and Pedal it is NOT Junk.
Who the heck told you that?
PSI uses the OEM ECU and GM Pedal. Harness is a harness. I put a PSI in my LS7 T56 car. A GM performance set up in a LS3/480 swap in a 69 with factory gauges and the GM Perf set up in my new 69 build with an LS3/525 crate motor.
http://www.autometer.com/tach-adapter.html
http://www.autometer.com/ls-install-kit.html
If using the factory tach it is easy to make work. Speedo no problem. You will need the metric adapters for the OP and Temp sensors. If mechanical OP then install the metric to SAE adapter and run the line to the gauge.
Hit me up if you need FREE help Lol
John