Been down this road already - search for my 3.9 to 5.9 swap thread, it has EVERYTHING you need to know.
You need the engine from anything, and a PCM
from a truck which is the same year as yours. That's extremely important. Engine doesn't care what it's in - as long as it's relatively new and low miles, you don't have to worry about compatability. If it's from a Ram, you'll need a Dakota oil pan.
That's IT.
42RE is fine behind the 5.9. A lot of people on this board talk about things that they have no experience with - they are just talking shit. Let them talk. Don't buy a new transmission. Get yourself a big trans cooler and don't worry about it. As the other guy said, I've had my 5.9 behind my 42RE for almost 40,000 miles and I race every weekend and street race at least once almost every day, and it hasn't shown any signs of starting to slip.
A few things you will need to modify:
-Your wiring harness. You will either need to get a harness for the 5.9, or modify your 3.9. IIRC, one of the sensors needs to be lengthened, and you need to wire up 2 injectors (easy as pie).
-Radiator hoses and heater hoses. They will be too short. Either get the right hoses, or get some clamps and extensions and extend them. I just extended what I had, and it's worked perfectly
-Your fan shroud. You definitely don't need a new one - If you look down into your engine compartment, you'll see the top of your fan shroud, then a big circle sticking out a few inches towards your engine. Take a sawzall and lop off that circle, and the 5.9 fits perfectly.
-One of your motor mounts. I forget which side it was, but if you use the mounts from your 3.9 (do it, save money), you will have to grind down a little bit off of one of the corners. You won't be able to bolt it to the engine - the corner of the mount will hit the block ever so slightly. Just grind it down until it fits like a glove.
Fuck the 9.25 rear end - Unless you plan on having a power adder, your 5.9 won't have enough torque to come anywhere close to harming your 8.25. If you're nuts like me, and decide to put 4.56's in the rear end, you aren't going to be able to keep your tires planted anyway :drive:
You don't need a new slip yoke. Not sure where that one came from or why the hell he said it. The back of your engine will be in the same spot as your 3.9, it will just come a few inches farther forward in the front where the 2 extra cylinders are. Thus, your transmission stays in the exact same location, and your slip yoke will rest in the same fine position as it was.
If for some reason, you feel the need to get a new transmission that is a different length then your 42RE (I don't know the dimensions, so if the 44 or 46 are shorter), then you will either need a new driveshaft or a longer slip yoke. That would be a huge problem - TRUST me, I learned that lesson in my Mustang. You need the slip yoke to slide fully onto the output shaft of your transmission. It needs to have a certain amount of that yoke on the output shaft, otherwise it doesn't function properly. There's a possiblity of the teeth on the gears stripping, but the bigger worry is rotation of the driveshaft. The U-joint is designed to allow the driveshaft and output shaft to rotate at different angles.. but it's still precise. If the driveshaft end orbits out of its axis of symmetry, your output shaft orbits as well, and so do the internals of your trans.. tears up the inside, governor bore hogs out the case, etc, and you can kiss your brand new transmission goodbye.
My advice: Keep your 42RE, get your hands on a low miles 5.9 out of a Dakota or a Ram, get yourself a PCM from a Ram of the same year as your truck (they have better PCMs then the Dakotas), drop it all in and fire it up. It's about as easy as engine swaps get. If you have any questions, drop me a PM and i'll help you.
Here's my V8. Take note of the way I fixed the radiator hoses and heater hoses, as well as the fan shroud modification.
You're going to be a happy camper the first time you drive your truck after the swap LOL. Man, that was a good day.
More pics, just because I love my cars. Here's my Mustang. She's sold now, unfortunately. I built that bitch from the ground up. Engine, trans, interior, body, suspension, EVERYTHING. Took me an entire summer, but it was worth it. Then school rolled up and said 'hey pay us tuition' so I had to sell her
And, just for shits and giggles, here's a picture of what my slip yoke looked like before my Uncle looked at it and informed me that it would be extremely bad to try and drive a car with any amount of power using that setup LOL. Hey man, I'm human too. Everyone makes mistakes - learn from them!
^^That's what you DONT want it to look like. Here's how I fixed the situation - I'd recommend that if you have the same problem, you get one of these bitches:
Lightweight aluminum racing driveshaft. Brand new U-joints and slip yoke, Custom made to the exact length I needed and balanced perfectly. Doesn't get better then that.