1st Start stock 340 w/ New or Rebuilt Carb?

If you don't have an electric fuel pump, go to a pharmacy and get a kids liquid medicine syringe. Fill the fuel bowls via vent tubes on a holley, pretty easy. Hit the squirter arms on both ends and make sure they are shooting fuel.

Yes on borrowing a good known set carb if available.

You don't want the engine cranking to pick up fuel via mechanical pump to fill the bowls and get fired.

JMO, set the initial timing in the 20* range, get it fired right away and up it to 35-40* at 2000-3000 rpm and watch oil pressure and water temps. DO NOT set it to your desired total number, it likely won't have enough timing and will get hot. No load on it, so a bunch of timing isn't going to hurt it.