Pistons should be matched to the stroke. The rod is the same and the crank stroke will push the wrist pin higher in the bore. If you are using a 4" (255) crank, you need the matching pistons for the 4" stroke. If you don't, you will have a bad day.
If you look at the selection from Summit Racing, you will see that the only stroke offered is 3.750. Here is the link.
http://www.summitracing.com/search/p...rd-flathead-v8 You need pistons for 4.000 stroke with the 4" crank.
Seek another vendor, maybe reds? I personally would go with a machinist that knows flatheads and let them buy the pistons according to my preference and do the machine work.