thats a great truck. '98 12v are the Holy grail Dodge. Just as the '07 5.9 cummins is with the non-egr. No matter what configuration it is with the '98 12v you get the upgraded interiors, 3+4th doors, and the best of the 12v engines. And personally after owning one I'll never own another '98.5-'02 24v VP44 powered cummins.
the 47RE/RH is a fine auto. Problem is no one EVER upgraded them when they put the free power to the P7100...........what do you expect. a stock 2nd gen 12.25" clutch won't hold up to it either. so what the difference. upgrade some auto parts or upgrade the clutch.
the front ends are known to get loose on them too. i love duallys. I daily drive a crewcab 3500. well until I took it off the road to put a 12v in it
If you put a few$$$into the 47RH as a billet converter and valvebody, before you hurt it. It'll take a bunch of power.
As its all been said, 15,5K is too high. check the breakout screw on the AFC housing. if its intact. either the pump has never been opened, or someone went to some pains to install a new breakout screw after they fooled with it. usually no one ever puts in a new breakout. just replace it with a metric bolt.
12v will not likely have a 53 block, except if it was a late model 12v in ;98. they were ramping up 53 blocks for the 24v. the problems there is some 53 castings had too much core shift during casting, and crack in the water jacket radius on the passenger side rear @ cylinder 5 - 6. if its an auto there will be the OEM auto heat exchanger right over top of where the crack would be. wouldn't worry too much about it but check the blocksides @ the panrail height near the front of the engine on both sides. if its a 53 there will be a large "53" cast on 1 side or the other or both. not all 53 fail. actually very small % of them do. but you always hear about the ones that fail. I put 606rwhp through my 53 block for a couple years. never a whimper. and I beat the piss out of if.
the killer dowl pin is a 100% easy, need to fix on any 12v. gearcase alignment dowl falls out into the geartrain A) nothing happens and it falls into the oil pan. B) catches a gear and gets shot out the timing case cause a major oil leak from the hole. C) gets caught between 2 gears and you have a catastrophic engine failure.