there's no one right way-----but there are many zillions of wrong ways---

If the car your talking about is the one in your avator---nice looking piece--would definitely be classic with either a flat head or hemi---but a Cummins????that engine and trans will not only be too long, too heavy,, but you won't be able to spin your tires , the sound will get you snoobed/blackballed at every gathering (cummins is even sicker sound than the import fart cans)

Do some measurements and don't forget the accessories--you will also probably need to front half your frame with the dodge truck frame(probably would be the easiest way to do that swap anyway--also do some weight calculations----

Now if you were going to pull an RV trailer, maybe you could mount the 40 body on a late model mopar frame-----