This may be elemental to some, but why are the rear spring perches wider on some than others?
Looking thru a stack, I noticed that some stick out about 4-5 inches from the frame rails. Others stick out about 8-10 inches. The frame rails look the same. Is there a difference in availability or price?

It sounds like you're describing the rear crossmember. The crossmembers, both front and rear, are wider on the '26/'27 cars than on the earlier models. The '26/'27s are a bit more scarce, and they fetch a slightly higher price.