rich-
I'm looking in my Chelsa catalogue right now.
There are four Zf choices and three corresponding Chelsa kits.
I'm going to type it verbatim.
TYPE /(RATIO) /KIT /PTO DRIVER
S5-42 (5.72/.076) ZF4 45tooth
S5-42 (6.56) ZF5 44tooth
S5-42 (7.5gas) ZF4 45tooth
S4-42 (7.55dsl) ZF6 47tooth
I'm afraid you're going to have to get dirty.
Only way to accurately tell is to pull the PTO cover and count the teeth on the gear. I've always had the tall 1st gear and it took me three gears to get through an intersection from a dead stop.
Also, remember you're going to need to get a new hyd pump. mime mount directly on the back of the PTO and has no aux. shafts. I've worked on units that had the aux shafts and they always scare me. Why? very easy to get hurt when the truck is stationary and the PTO is engaged with the motor running. I'll keep my info with me tomorrow just in case you want me to fax the corresponding pages.
good luck!