to make a long story short, we are swapping an engine from a 75 chev truck into a 65. The wire going to the hei from the donor truck is about 12 awg, and the wire in the 65 for the coil is about 16 awg. Is this a big difference? What size wire should we use. And what size fuse?

 

In '82 - '92 GM used a fusible linkfed from the large lug on the starter to supply power to the ignition switch. There is no fuse between the ign switch and the coil. I'd use #12 wire between the battery (or from the large lug on the starter) and the ign switch and #12 between the ign switch and the coil, as well. Fusible links are available from most auto parts stores (Pep Boys sells them), however, you may use a high amperage fuse, like 60 amps, or so, if you would rather use a fuse. The power feed to the ignition switch feeds other circuits, as well, so the current draw on that circuit will be greater than just the ignition coil. There are some wiring diagrams on line at:

http://www.geocities.com/vtcamaro/Wire-Diagrams.html