IMO, a double pumper is a waste on a streetable car. YOur cam isn't really big enough to require one.

You have to tune the idle circuit and I am not talking about the idle mixture screws here. There are innumerable books out there on how to tune a Holley, get one, read it, ask us questions till you understand it then buy a wideband O2 sensor and tune the Holley.

You have to understand that ANY double pumper is going to have a substantially richer out of the box idle circuit than a vacuum secondary Holley, it's how they are built. When you use a double pumper in it's factory intended role it works. When you use it in your application it doesn't work so great.

You are going to have to tune the idle feed restriction and idle air bleeds to fix this.

Oh, a 750 might be a bit too big for a 360, but it isn't so far off that it isn't useable with some tuning. you might get a bit better mileage and a hair crisper low speed out of a 600-650, but the 750 will likely improve your topend. Depends on what you want.