Truck Repair: smallest cummins bolt in/reverse hotrodding, rearend gears, stepvans

Question
What is the smallest diesel that will bolt into a Dodge without fabricating custom mounts etc.?  Some came with cummins inline 6 engines and I hear a few delivery stepvans had similar but 4 cylender engines that might bolt up.  Is this true?  I work with people interested in biodiesel but we have trouble finding diesels that are small enough to maximize city mileage.  We are totally uninterested in speed and wish to run with the bicycles.
Also, what is the smallest gas engine that will bolt up to a bellhousing that formerly served a 318 Dodge? Is it 239/3.8 or can I go to a 3.0, 2.5 or 2.2, (with lower rearend gears)??
Also, can the engine and tranny from a 2.5, 2wd Dakota be bolted into a B100 van without fabricating mounts? How hard is it to fabricate mounts? Other problems?

Answer
There is a 4 cyl B series engine that everything bolts up the same except it has 4 cyl.Not all parts are the same like oil pump & oil cooler but bellhousings & motor mounts will bolt the same as 6 cyl.The frt of the engine is the same except dampner is different.The only thing is it will have to have the manual engine stuff(pre 24V) Dodge componants if you are wanting it all Dodge looking but original Cummins componants will work.
The next thing is you want to keep the Dodge tranny then find a donor wrecked PU that has the auto & get the adaptor or can purchase new one from Cummins(Dodge will be higher,but call for price shopping).Anything else let me know.