There's no way I would attempt this if you don't have access to the car.
There's going to be variations even between two identically-equipped cars, and between different tastes as far as how closely-fitting you want the hole to be. Someone else may have different linkages, fuel line setups, etc. They may cut the hole looser to give more clearance for engine movement if they're using rubber engine mounts; or tighter if they're using solids, elephant ears or a motor plate. There's just no substitute for doing your own measuring and fitting.
Last time I cut a hood, I did a mock-up first. I bought a 4x8 sheet of 1/8" Masonite (hardboard) and laid the hood on top of it (it wasn't big enough for the whole hood, so I just did the rear 2/3) and traced the hood outline onto it, and cut out the Masonite to the hood size. Sinct the hood has a crown to it, I screwed on a pair of arched cut 1x2s towards the front and rear of the "fake hood". It was now a sufficiently accurate template of the hood, that I could set in place on the car for fitting. I started by locating the two centers of the carbs and opening up only enough for the carbs to begin to poke through, the gradually opening it up more and more and massaging the shape until I had it where I wanted it- if I cut too much out, I'd just duct tape some cardboard over the booboo and re-trim it where I wanted it.
When you're done sizing the hole, remove the two 1x2 reinforcing ribs and lay the template on top of your hood, and transfer the shape of the cutout to the surface of your hood with a pencil, sharpie, scratch awl, whatever. You can even use a spray bomb and use the template like a stencil.
Then you can cut your hood out and rest assured that the hole is going to fit like you want it to, with contours that are appropriate. Sounds like a lot of work, but you only get one shot at this.
A couple of things to keep in mind: the hole is NOT going to be centered. The engine is offset to the passenger side 1 1/2 - 2". If you have a center rib on the hood, the hole will have an obvious offset to the passenger side. This is aggravated by the fact that you often need to cut the hole with additional clearance on the passenger side to accommodate engine movement, and the offset becomes almost glaring. Some people cut additional metal on the driver's side to even things up, some just throw a scoop over it, and others don't really care one way or the other and just live with it.
