You basically have the right idea in your first bullet. The simple approach is to meter and set your exposure for the foreground. Then meter the sky (not the sun), and pick a ND grad filter strength that is the difference of the two meter readings (or slightly less, to within a stop if you can).

Depending on your scene, you might need to adjust your exposure slightly. For instance, if you have deep shadows in the foreground (perhaps from nearby mountains or trees), you'd probably want to increase your exposure a little bit. However, if the foreground is primarily reflective water and you don't have deep shadows, I'd definitely decrease the exposure a little.

Lee Filters' page on ND grad filters has several examples with details about filters used, and a section about metering. I found it helpful when I first started getting into shooting with filters.

I definitely recommend experimenting with your digital camera before committing to the Mamiya. But note that depending on the size of your digital camera's sensor and the lens used, the filter will appear to be a softer transition than when it is mounted on the Mamiya. The same hard-transition ND grad filter on 6×7 will appear more like a medium-transition ND grad, or even soft-transition, on a DSLR or MILC. Especially so on a crop-sensor body. You can reduce this effect slightly by making sure the digital camera's lens projects a similar field of view as the Mamiya and its lens.