In my limited knowledge I will say a 1st gen I/C 6bt is the most wanted motor.

BUT there are other that will disagree. 

I don't like the elec controlled motors, and I think the 24V chatter too much. 

For me simpler is better. 

I'm at work, so I'll make it short.

I have a 90' Cummin's Non-IC. I couldn't ask for a more "powerful" truck, and even with about 250,000 on the clock, it's got more get up and go than any small block I've ever driven, and is quite often "quicker" than a lot of newer trucks. I prefer driving my Cummins over my 2004 Ram.

Tweak your pump ("free" power)
Invest in Guages (EGT, Boost, and Fuel Pressure is a good one to have)
Have Fun!

One downside: if it's an auto, it will be a 727 trans. The Cummins could really use the OD of a A518.

I sent a fan through my stocker, and I went ahead and replaced it with an aluminum one from Summit Racing. Check out McCullogh Fabricating http://www.alumrad.com I had some parts for my radiator sent from them, and they were GREAT to work with.

Bought my T-stat housing that sends the hose to the drivers side from Cummins NW. All in all it was about $500 (my shipping is a little rediculous, being in Alaska) kept the stock fan, tried going with an Electric fan, but I didn't have enough room without messing with the core support.

If I were doing a 1st Gen engine swap I wouldn't hesitate to go with the Non I/C engine but I would buy a 2nd Gen I/C and put that in as well.  The 89-91.5' engines have injectors that flow nearly 20% more fuel than the later 91.5-93' 1st Gens.  The radiator and lack of automatic OD transmission is the downfall.  The head design was improved in 91.5' but that doesn't mean all pre 91.5's heads should be condemned.  My 89' has over 300k and still running strong, if it cracks too bad to use I would find another rather than go with an improved one.  Not a big issue IMO.

I'm not sure yet Joe I have no clue what the PO did to it. I'm pretty sure the pump timing was changed, as far as the pump I'll find out when we take it off to do the reseal. 

And no I don;t have any air mods accept for the K&N the kid put on it. 

Mechanical temp gauge that's it. I haven't gotten to the point of putting in a EGT gauge yet.

I have a second trans cooler on it. 

I'm going to reseal the IP next week since it's leaking and we'll know where the pump was set at. It blows black smoke even at 60MPH when you stomp it down.  I'm thinking the guy cranked the fuel screw in the whole way. 

If you do find yourself getting warm, pull over and keep truck in neutral high idle, about 1000rpm maybe a little more, pop the hood open. Within 5 minutes you will drop major temp unless something has failed. Make sure not to shut engine off though. I blew a radiator in half that way. (end tank ripped clean off)

Hit about 204 once, the engine had basically been running for about 18hours down the highway at that point, 6000ish pounds, through the ozarks. 95* outside, air conditioning been running the whole time.

Now you can see why overdrive may be of assistance. And mad max's air intake.  

I back off when I need to, like as it gets up around 195-200*. Once on the flat it cools quickly. SO I'm figuring it a fuel issue on top of the down flow rad. 

Pulling down the highway at 60-65 it runs at 185-190* on a hot day.

But even in 30* temps it will still jump up to 200* pulling a hill.

What I have read and heard IIRC the 6bt weighs in at 1200lbs. A 440 weighs in at around 600-700

Hey Dave, great project you have going on.  Not sure if you've seen my thread on 'Nacho', but the diesel swap I did to that 78 RC sounds a lot like what you are planning.  The diesel swap made the truck feel night and day better than the 440 that was in there, and even though the truck was pretty hefty (just over 7000 lbs) it was extremely sure-footed.  In the end Nacho had 36" tires, 3.54 gears, a '93 6bt/518, a 203/205 doubler.  It was fantastic.  The Cummins revs up and down faster than a gasser because it's 17.5-to-1 compression, and like you said it will 'run' upside down or any angle.

Weight is certainly heavier on the diesel, but the torque gained seems to make up for it nicely.  I would suggest some fat tires - not necessarily tall, but the Mickey Thompson 36x15.5's are a great sized tire under that kind of weight, especially up front.  The weight is managable but you need the right hardware in the right places, and lockers, especially up front, will definitely make 'wheeling a lot more managable.  

Also, you won't 'need' any power upgrades at all to have big fun wheeling.  You could leave the engine stock and spend the $ on chassis upgrades. 

If yer looking for some vids of a diesel RC build for wheelin' I'd offer ya to have a gander at a couple of the vids of my old trail rig 'Nacho', a couple years ago at Moab - you can almost 'feel' the torque .  Remember this was with 3.54 gears and a doubler (4-1 final ratio).

reply # 2131: http://ramchargercentral.com/index.php/topic,49434.2100.html

The rig I have in progress 'Bud' will have 4.56 gears and the same doubler - that build in over in the 'trucks' section.  Might also glean some ideas.  Any questions ask away 

- Sam

yah the last couple thread pages sum up the next 'evolution' of El Nacho, and the generation of the new project rig, Bud.  It's all been good fun

And regarding committment issues, the 440 I replaced was only between the fenders for about 6 months, and once the diesel got in there I never looked back.  Mechanical direct injection, stable reliable cooling, and mountains of pure smooth torque make for my ideal trail rig power plant.  You won't need a lot of power, and one thing the 6bt's enjoy more than the 4bt's is the availability of aftermarket parts.....and stock parts for that matter.  Not a lot of 4bt's around...