I have a 98 chevy 2500 4x4 with a 350 with 258k on it. The truck is in excellent shape but the engine is getting tired. I came across a 8.1 out of a 01 RV with 3900 miles on it for a good price with the 4l80e on it. I am trying to see how big of a deal this swap would be. I have the brain box,throttle sensor,o2 sensors, and complete engine wiring harness. The firewall conecters seem to be very simular. Just need some ideas or thoughts on this project. Thank you for any help I can get.  

It really should not be hard at all and should bolt in with little effort. A 8.1 is based on a GM BB or 454 so fit is not a issue as that model year could be had with a 454. One issue I do see though is maybe engine height because 8.1's have more fairing and stuff on top of engine an 01 on trucks had raised hoods which were raise to accommodate Dmax but gas engine used this extra room some too. You want to upgrade radiator too as do not try to cool it with stock 350 one and also the 8.1 MH engine should have a 4L90e attached to it not a 4L80. The 4L90 (which many do not know it even exists)was a beefed up 4L80 designed to work with BB's in MH chassis's.  

I am not worried about the physical unit bolting in, like you said it is still a BB. Where does a fellow find a 4L90E? Would a built up 4L80 work? I am more concerned with the electrical side of the swap. I am not affraid to try things that most people wouldn't think of but I use this truck daily and want to do the right thing.  

If MH had tranny on it it would be a 4L90 (they look the same) There was also a 4L85 starting in 05 but you can retro fits any 4L80 with upgraded 4L85/90 parts. Not that a 4L80 is weak as it is "rated" at 440 ft lbs input torque, 85 at 465ft lbs and 90 at about 490 ft lbs.  A 4L80 is nothing more than a THM400/3L80 with a OD added to it and a electronic valve body (there was also many THM375, THM425 and THM475's that were different builds of basic THM400 and many do not know this and assume they are all THM400's when they are not) The 4L90 can be found in big panel vans with BB's (like say Snap On tool trucks) The electrical side is a issue for sure but if you have all the wiring and ECM for 8.1, I do not see any big problems there. Fuel pressure is not a issue and it is basically same as 350 VorTec.

I think this is a neat and sensible project and would like to see how it works out for you. It would make a nice "sleeper".  

I wonder if I can use the input shaft from my current 4L80 in the 4L90 to make the t-case work. I was told that I could swap shafts to make work on my t-case.I would like to use the tranny that is mated to the 8.1 if at all possible because it to only has 3900 miles on it. I am glad to know the tranny is better than I thought. Is there any way to be sure,any numbers or codes,or measurements I could find? I'm sorry but what does the MH stand for in previos conversation? Thank you for all your help and advice.  

motor home  

Tail shafts should swap out fine. The main reason they came up with Ally was because they need more ratio range for Dmax's limited power curve (drive one with a stick as I did once and you will know what I am talking about) not because 4L80 could not be beefed up to handle it. (they actually used a version of a 4L90 in Dmax powered vans GM made) My only concern about using 4L90 you have is the case thickness. I do not know if it is a issue with them but it is with out GM trannies as cases destined for 4x4 usage are thicker and stronger to withstand Tcase counter torque for Tcase bolted to it. Beefed up 4x4 cases generally has a heavy cast bolt on lower torque converter cover while 2WD case had one of thin stamped steel.  

Sounds sorta cool, but a '79 454 into, say -- a 1987 deuce would be much more cooler.  

naked.farmer:
Sounds sorta cool, but a '79 454 into, say -- a 1987 deuce would be much more cooler.

You mean a smog control free 454 LS7 crate motor don't you? It was conservatively rated at 500 HP. A 79 454 was quite weak in comparison with about 1/2 the HP.