How many cups is 8 whole peaches equivalent to?
If I gave you any number I'd be almost certainly wrong - peaches come in many different sizes. (And personally I'm led to believe that any recipe that gives only a number like "8 peaches" is either crap because is's imprecise or tolerates a lot of difference.)

So I suggest you


either find a recipe with a weight
My first guess is jams that often use equal or specific-ratio parts (by weight) of ingredients.
or one that gives volumetric meassurement for the cut fruit
"add two cups of sliced peaches"
or choose something that can be imprecise because you add ingredients to taste.
Compote, for example. Less sugar for ripe fruit, more for tart ones...


And if all fails and you have only sliced your fruit, try to "build" a whole peach (sort of, you don't have to find the original pieces, this isn't CSI) and weigh that. The rest is simple math. If you want to be very precise you could weigh 1c sugar as well and subtract from the total amount, but for 1/2 bushel of fruit I'd probably ignore it.