Just a heads up there are a 4 and 6 Series Fuller two speed and a 5831 auxiliary transmissions on E-bay. As I remember the 6 Series two speed is a little smaller than a 5831-X box. 2.30 and 1.93 under drive. Never seen a 4 series in person but the Spicer documents it is lighter duty.
https://www.ebay.com/sch/i.html?_nkw=au ... 4084.l1313