How much safe power on a 1996 CTD 5.7?

I have a customer, car friend, who has a rolled over1996 CTD Ram 2500 long bed extended cab pickup that he wants to use in a 1985 Ram Charger body. We talked about mounting the Ram Charger body on the pick up frame by shortening the pick up frame, he is up for that and several local shops can supposedly do that safely
My question is how much power can he safely have added to that motor turned up for street use? He doesn't know the gear ratio in that truck but he wants 36 or 37 inch tall tires on all four corners.
I haven't paid much attention to the early 12 valve motors as my first CTD was a 24 valve 1998 extended cab 2 wheel drive, my current truck is a single wheel 2006 Ram 3500 4x4 automatic. I liked both of them, especially when towing my loaded 24 ft. enclosed Pace race car trailer going to the drag races
The 4x4 became necessary when we moved up to central Oregon with the ice and mud in the winter and spring

How much power is safe for what? The chassis, the transmission (BTW, you didn't say what the trans is), the rear end? The P pump engines can pumped up a lot. I would consider a billet cover torque converter for an automatic or a clutch upgrade for a stick at a minimum.

He may have to change the Ramcharger firewall, and install one from an 89-93 pickup that came from the factory with a Cummins. I have a friend here locally that built an 85 crew cab Dodge, and installed a 12 valve 5.9. He had to do the firewall swap, and I think he had to do something different with the radiator support also.

There have been plenty of Cummins swapped into the earlier trucks without changing the firewall. The core support is different though. On the 96 the engine sits further back in the chassis than the 1st gen trucks do so to get it too fit using the 96 chassis the front wheels wouldn't be centered in the opening or you would need to modify the firewall.

I have personally swapped a Cummins into a '83 D350 and did not have to swap out the firewall. I did however have to change the electrical bulkhead opening to accommodate the '92 electrical harness. If you have an intercooler it is easier to swap the core support but you can remove the intercooler mounts from the 92-93 and weld them on to whatever one that is chosen. Its as easy as swapping from a SB to a BB especially if you have the donor truck

He may have swapped the firewall for other reasons, if it isn't necessary.....I just remember him saying he swapped it. I know he had to do something to the core support because the later diesel truck he got it out of had square headlights...and his truck has a 76, 77 or so grill with round headlights.

Not sure about max power for street use but I used to run around on 40s with my 12v and 4.10s. Ran 13.0s on super swampers so I am assuming power was in the 500s+ at the rear. Would wheel and tow with the truck. Only thing I ever broke was my output shaft and motor mounts.