Mixture has to be close enough to run for 20 minutes without overheating.
If running hot, or headers turning red: indicates severe lean condition
Ignition must work and timing has to be quickly adjustable , also no overheating.
leave dist holddown bolt snug but not tight at first. If timing within reason then tighten.
During those 20 minutes do not keep a steady rpm. Blip the carb periodically. Helps to get more splash (oil)
around lifter cavity area.
SO.
How do you KNOW if brand new untested carb and ignition parts will actually work ?
do you mean New but Untested?.
There are various checks U should do on ANY carb, new even rebuilt.
I don't think you can know that.
I have an OLD orange ignition box that worked 25 years ago. But I am thinking about firing the engine with a points distributor / coil, one that has been
checked on an old distributor machine and has a known advance curve. If the points gap is set, it should fire the engine, right ?
If you're already wired for that box, then use it. You'll know right off it works OK.
On the carb, if I put on a brand new Edelbrock/Carter and go through it first to make sure there is no debris in it, it should fire the engine, right ?
What else am I missing ?
Always pays to have a 2nd pair of eyes at fire up. Too many important things to monitor during those minutes.
One may be adjusting carb/timing, while other checks for leaks/etc (top & bottom. Listening for odd sounds.
