Re: what year cummins are the good ones and the bad ones
All of the engines from the "1st Gen" Dodge (1989+) to around 98 had the "Killer Dowel Pin" which is an easy fix. Yes, a 24v head will fit on a 12v block(blocks are/ nearly are identical). It would be easier to get a 24V engine and hang a mechanical pump on it converting the engine back to all mechanical than trying to put a 24v head on a 12v engine.
There are also basically two different versions of the 12v that are found in Dodge trucks. The early engines had a Bosch VE rotary pump. This is a good pump and driveability is excellent, but the pump physically is too small to pump enough #2 to support more than about 325HP +/- 25. The second version of the 12v is more commonly known engine with the P-7100 inline pump. Of any engine the desirable components are the ones cast in the Brazil foundry. Although any non-53 block is of a decent quality, the Brazilian castings are a bit heavier than other versions.
Although I've had several Cummins 6BT powered trucks, two factory Cummins powered Fords, I still like my 7.3 and will continue to run it. The 6BT's are a good engine for sure, but they have their problems too. 53 block to name one, head cracking is another. If you want to put a 6BT in a Ford it's getting fairly easy to do. If you want cheap HP get a CR and spend the time (if yours it's free) to get all the electronics to work. The 12v is a simple and reliable engine and can make very big HP reliably, but it will cost more.