First off, I hate you for getting a P-Pump motor for a grand!!!
A 12V will fit in a GM frame with no suspension lift, in case anyone was wondering. You don't need any body lift to make it fit either.
Scott Oswalt, owner of TN Diesel Conversions makes a crossmember and motor mount kit, which is what I'm using. While they are somewhat pricey, you can have them the same week. He keeps them in stock and is also a Cummins dealer; I deal with him frequently. Also, any time I have ever called him for "tech support" he has always answered and walked me through my problem, even on a Sunday.
A good reason not to use a Duece trans is ease and availability of parts, plus it's really heavy and adapting it to a transfer case would be a pain. Then you have to get the clutch hydraulics to function, etc. Too much work...plus, SAE bellhousings are huge and there's not a whole lot of room to work with in a GM transmission tunnel when doing this swap.
I have a 3200 rpm governor spring in my VE pump. Works a lot better than the stock 2400. Way easier to cover the gearing gap between 3rd and 4th on an NV4500 and I can make some nice boost (a hair over 30 lbs). With 4.56s and 39s, I can easily run down the highway with traffic (faster too) and net some pretty good fuel milage.
What else? Umm...
Oh, if you use the 34" core radiator from the 6.2, you will severely over-cool a Cummins. In addition, the intercooler will not fit around it and getting the radiator hoses to fit is a pain in and of itself. 2nd gen intercoolers (IE: P-pump motor 94-98.5) fit better and you really don't have to trim much out of the core support.
Lastly, there are no computers. All 12V Cummins motors are mechanical, however, in certain applications such as a Dodge truck, they used a computer to control the alternator's output and the transmission. This is not needed to make it function as you can use a Dodge style external voltage regulator, but you will need to add a capacitor in the keyed wire to keep the lights from pulsing OR you can do what I did after you get tired of that crap and get up with PA Performance for one of their 1-wire conversion alternators. You can get a 136 amp or a 200 amp hi-output alternator. I also got a 4 ga power wire kit with a 200 amp mini-fuse holder. No more charging problems what-so-ever, plus more amperage than the stock 120. Again, expensive parts, but they're worth it and they last a while.
I suggest using everything from under the hood of the Dodge. Radiator, intercooler, etc, etc. Makes everything fit better and work likes it's supposed to. Also, first gen (89-93) exhaust fits much better because of the downpipe. I was running a 2nd gen exhaust, but the downpipe was on the firewall, so I copied the design of the first gen downpipe and made my own.
If you have any more questions or concerns, feel free to ask. If I can't answer them, I'll put you in contact with some people than can!