pully sizes 5.0

[greydan]

I've installed a new serpintine belt on my '87 GT. Had to take several home to find one that fit. At first I figured the parts book was wrong??? but it's occured to me (dah) that the thing might have underdrive pullys on it. Trying to find out what size the origional pullies are is like pulling teeth..no one knows and the dealer just walks away.... The local speed shop, won't open the box, to tell me what size the underdrive pullies are either....just smerks.
There isn't a 5.0 in a salvage yard with-in 100 miles of here.
Anyone know the dimensions of either, stock or underdrive?
I was planning on installing a set (Santa Claus) but it appears I might already have them on the thing.