I'm quite aways from having my engine completed but just wondering if there is any big advantage to pre running it before installing. It will be on carbs.

Absolutely!

Especially a fresh engine!

Bunch easier to re-adjust valves and re-torque head nuts after first full heat cycle if the puppy is out of chassis.

Oil leaks are easier to spot/fix.

Good/bad memories will be made...

Kinda what i was thinking, exactly.
I guess it cant be that difficult to hook up a temp fuel pump, battery, etc.
The transmission would be attached anyway before installing into the car.
Is there anything I'm missing?

We run almost all our engines on a test stand before installing in the car. Easier to spot leaks, noises, etc. you can put a 914 engine and trans on a pallet and run it. AIM a fan at the engine if you run it for more than a minute

Yes, a couple ratchet straps and a pallet works.

You should remove the alternator belt (or hook up a voltage regulator temporarily), as you'll not want to potentially destroy the alternator.

Do you do the cam break in as well ?

This guy makes some stuff that helps. Maybe if you are only doing one, not worth it...
https://zalexindustries.com/

In addition, I prime the oil system by cranking it over to achieve oil pressure before starting a new engine.

keep us posted with your progress