Alternator Upgrade and Bulkhead Disconnect

I've grown oddly attached to my '67 Dodge Dart 270 (225cid slant-6), and I've decided to get it resprayed. I'm in the process of taking the car apart to prep it for the new paint job. I pulled the wiring from under the hood and removed the cheap-o electrical tape that Dodge put on the vehicle fifty years ago. Having inspected the wire, the only one that needs to be replaced is the temperature gauge sending unit's wire. (Some yahoo had it running under the rocker cover when I bought it, and the gauge didn't work.)

I've decided to perform the alternator upgrade recommended at Allpar for a 60amp alternator. I'd like to be able to tap into the cigarette lighter to charge my cell phone and run a GPS at night (if such were necessary). Here's my question --

They insist on going through the firewall itself for the red and black wires. Why am I not able to replace the wires that currently exist that run to the ammeter through the bulkhead disconnect? I have the engine bay wiring out, and I'll soon have the under-dash wiring out ... so why can't I just replace the wires that are currently there?

After that is explained ... what does the author mean by using a squeeze-tap connector? Are there any pictures of that (which would be most helpful, since I'm a visual learner)?

Thanks!