what box is on the right in your pic?
i have been looking at boxes to do the conversion on my cj and every box i come across measures 3 7/8". i went to my WD supplier for my work and checked out and measured 18 gear boxes for jeeps, chevys, buicks etc, you name it and they all measured 3 7/8". i have yet to find one that is small like the one in the picture.
so your pic of the large and small gear box has me wondering.
and so everyone knows, every saginaw gear box will bolt into any vehicle (that uses saginaw box) as long as
1.the input shaft splines are correct
2.the fittings are the corect style (flare or o-ring)
3.check if you need 3 or 4 mounting bolt holes
all three of these boxes had a 3/4 30 spline input shaft on them
the pitman arm shaft is the same size and spline count on every power box i checked.
So is there another way to id a J-20 box? i have bought 3 boxes in the last 2 days and honestly i cant see any major external differences between
the 78 j-20 box had flare fittings and had the nipple on the input shaft
the 85 j-20 box had o-ring fitting and the nipple on the input shaft
the 90 cherokee/wagoneer box o-ring fittings and no nipple on the input shaft