I just installed an american wire kit for my 1968 firebird. it was very straight forward and if i had any questions they were there to answer them. I bought my kit from ebay 499.99 just type in search on ebay wire harness kit for 68 firbird. in closing stay away from painless wire. there were anything but painless.
I used the Painless. There were a couple of problems that I had to rework, and the Tech support was useless in figuring them out, but I was able to fix them with some trial and error, a multimeter and the wiring manual. Other than that, it's a great system. Lots of circuits, easily identifiable wiring, easy installation. I would probably use it again if the need ever arose. FYI, the problem circuits were the wiper/washer (wires switched, blew a few fuses until figured out) and ignition switch (no juice to coil while cranking, wire missing).