OK, so far I can't find the length of the different models in my books, but have a few more to check...
Looking at the pictures in the manuals it probably is a Spicer 6xxx or a 7xxx. They have the same sort of mounting ears as seen on yours, and a 5831 is defiantly different.
A 7xxx Spicer is really big, probably looks like it should not even fit, a 6xxx Spicer could look at home in your truck...
There really should be a tag on your transmission, and if not then you will find the two rivet or screw holes where it is missing. Try looking about 1 or 2" below the top on the side near a corner, start at the left front and then left rear.
Grigg
