Yeesh, can be done, but its pretty much a gut and replace job electrically.
The clusters are different although less so when talking an ODBI gasser and an OBDI diesel. But that's the least of your troubles.
93 6.5 is mechanical injection and the 5 speed has no TCM.
The 89 will have an ecm to run the fuel injection and it will have some control of the transmission if its an auto (lockup is pretty much it).
So, pretty much all the engine and cab wiring is going to have to transfer over. The 5 speed will work just fine without input from the 7477 ecm, but you will likely have a check engine light since the ecm won't see the auto transmission. You can either live with it, find a chip from a similarly equipped truck or have one custom burnt.
The engine bay is also pretty much strip it out down to the engine mounts and swap the gasser stuff over.
Fuel system from the tank forward will have to change. Diesels don't have an in tank fuel pump, just a low pressure lift pump on then frame rail. Not enough to run fuel injection. You may be able too get away with a high pressure pump in place of the lift pump and figure a way to hook up the pressure and return lines at the TBI. Finding an external fuel pump that can pull form the tank as well as push to the injectors might be a bit of an issue. OEM fuel pumps are "in-tank" mounted so they only have to push. Aftermarket pump should be able to fix you up for an external.
There's so much that needs to be done, you're just going to have to get into it an do it.
Line 'em up side by side and start moving stuff over is about the best advice I can offer....take pictures as you take things apart. Not so you can post 'em, but so you can reference them while putting stuff back together.
