A couple of quick thoughts: as you say that is a lot of digging and a lot of soil to dispose of so don't forget that part. You may want to check around for a guy with a bob cat or small excavator who will do the job in one hundredth of the time it will take you. What will the eventual width of your path be?

The path width would be 32". That's why I figure the base should be 40" wide.
What is a "small excavator"?
Thanks.

Unless you really love hard manual labour, and have a lot of spare time on your hands, hire someone, and save yourself the aggravation.

Check your phone book to find someone with equipment that narrow. By the measurements you've given, it probably will take them, at the most, an hour. Plus, they'll haul away the dirt and rocks. You'll be charged a minimum fee that will be well worth it.

Dumb question but why are you going down 6 inches? Pavers are only couple of inches so you want path lower than yard, it will be sitting in water when it rains, won't it? Any reason you can't just be sure it is level, put weed cloth & pavers & edging & work sand in. Just asking? That's what I did & it has been a year now & no weeds have come up & I no longer get my shoes wet & can go to side yard without having to change my shoes. Mine is small patio about 7x10 ft. under an orange tree-tree is best it ever has been, lots of oranges & covered with more blossoms.