What you must do before you hook up your 12 volts is to put a resistor in the fuel gauge line. 12 volts going through that will fry it in a milisecond!
I think the idea of wiring the coil to a toggle switch may have been an attempt at theft prevention. Its an old trick but it works.
As to rewiring I have done mine without a wiring kit. If you have the schematic you are in good shape. On it there are specs for the wire thickness (gauge) and its for 6 volt. You can use that gauge for a 12 volt or even go one size smaller.
The wiring on my 49 Chevy was pretty simple. I had the original wires so it was easier than what you have to face. I started at the fuse box which is different to yours and went forward to the lights, etc. From there I just did one wire at a time. Good crimped connectors (a good crimping tool is a must) and cleaned up the posts I was attaching to and in some cases put heat shrink over the terminals. Do not solder your connections as vibration could make them fail. You will be surprised how easy it is - until you have to crawl upside down to get to those gauges under the dash.

1949 Chevy 1/2-Ton
"Sedgewick"
In the Gallery
1989 Caprice