'65 Valiant No High Beam Problem

dibbons said
I have low beam, but when pressing the floor switch for high beam the headlights go out and the ammeter goes to full discharge. I wonder where the first place to look would be? Thank you.

This is clearly some sort of serious short. You must find it.
Since it only occurs when the high beam is turned on, it is almost certainly in the wiring from the hi/lo switch to the headlamps. However...

dibbons said
Update: With the headlamp switch disconnected and removed from the dash, I started the vehicle and ran it for 15 minutes. The ammeter worked fine (shows more charge at first then less charge), as well as the brake lights, horn, front and rear turn signals, dome light. My multimeter is on the blink, so I need to replace that first to track down any shorts (working alone).

Its highly unlikley that its in the headlight switch since it does not happen on low beam . The only point of concern is that the circuit breaker in the headlight switch should have opened as soon as the current exceeded 15 amps. The first thing I would check is every location where headlight harness goes through or next to metal edges. Headlight buckets for example got me (on my 67). Relays are a great help but do not prevent this sort of problem.

dibbons said
Reader beware: This comparison I found online may or may not be accurate.

View attachment 1716171021

That appears to be roughly accurate although I beleive the author may have been using H6024 lamps. You would have to provide the details. Regardless, what @67Dart273 just posted is worth restating. a) Headlight relays are worthwile for several reasons. b) Its not revelant to the issue you need to diagnose.