I can see why your locals are saying go big. You can pretty easily ($12k ROM) get 500 hp to the rear wheels with a 548-572 BBC and still run pump gas. No bottle needed either. To do that with a 350 with twin turbos is gonna be fraught with issues IMO. And that engine will likely wind up costing more than the naturally aspirated BBC on pump gas. And it will not have the torque curve anywhere near the BBC in the low and mid range.

All that said you probably want to start the CR at 8:1. You are going to need the best of all parts in all engine areas as well. I don't know of anyone producing twin turbo kits for a '72 Monte so there will be a lot of engineering and customizing required along the way.

Kitz