If possible at all, can you get a picture of the SPID label in your glovebox? The 1988-1994 models unfortunately often have illegible labels because they're on the floor of the glovebox
but in case we might be able to make out the RPO code for the transmission, that might shed more light.
It would be really uncommon to see a TH700R4 in an 8 lug 2500, that's why I brought up the possibility of it being a 6 lug. It's entirely possible one also got swapped in there over the lifespan of the truck. They're inarguably a light duty transmission and not up to the job you want to throw at it.
You can quickly identify the 700R4 by several features. The pan is pretty much square, has 16 bolts (so does the 700's younger brothers - 4L60/4L60E) - the 700 will have a TV (throttle valve) cable that is critical to line pressure and shift behavior, this cable attaches at the throttle crank on the side of your TBI and runs down to the passenger side of the case just above the pan, attaches with a single bolt. Driver side has a domed sheetmetal governor cover just ahead of the tailshaft housing. Some google image searches will help you identify whether that's really what you have in there.
Richard