* The specific 350: Different 350s have different accessory placements (alternator, power steering pump, etc.). Some configurations might require modifications to the Monte Carlo's engine bay to accommodate them.
* The existing engine: If you're replacing an existing engine, it's much easier. If you're installing one from scratch, you'll need the transmission, mounts, exhaust system, and wiring harness that are compatible.
* Modifications: While it's generally a bolt-in swap (meaning it fits without extensive modifications), you might need to modify or replace some components like the motor mounts, exhaust system, or possibly even the firewall depending on the specific engine and accessories.
In short: It's *likely* to fit, but not guaranteed without some potential minor modifications. If you're doing the swap yourself, research the specifics of the 350 engine you have and compare it to the stock engine bay of a '79 Monte Carlo. Consulting with experienced mechanics or online forums dedicated to classic Chevy muscle cars will be very helpful.
