LH6/6L80 into a 72 K25

[CRMW]

I've been gathering parts for a few years but its now time to do the ls swap into my old GMC. When I first got the trucks there wasn't very much info available on the 6l80 swap and a few other projects got worked on first, although i would periodically do some internet surfing as ask a few questions.

The Truck

1972 GMC K25 -
I picked it up a couple of years ago, its factory black and white deluxe two tone paint, mostly stock but was used on a ranch for the first 45 years of its life and had saddle tanks installed just to get it to the end of the driveway as it has 4:10's.

The sacrifice

2009 GMC Sierra 1500 4x4
6lH - aluminium block 5.3l
6l80 - Transmission

I like the idea of the 6l80 with the 4:10's, I have a 350 in my C10 with 4:10's and it just screams going down the road so hopefully the double overdrive will help with that and the gas mileage.

Now I seem to have another issue, too many threads and I'm having trouble finding a detailed "how i did it" thread among the results.

Im gonna keep on looking but if anyone knows of a thread, or has any ideas for me, please let me know.

Chris

[mongocanfly]

The LS swaps are fairly straight forward...you'll need to adapt the 6l80 to your transfer case...other than that the install is really no different than any other...

[YoungPup1977]

Originally Posted by mongocanfly

The LS swaps are fairly straight forward...you'll need to adapt the 6l80 to your transfer case...other than that the install is really no different than any other...

when you state he has to adapt the 6L80 to transfer case what do you mean ? He already has a transfer case from the donor truck that is already bolted up to the 6L80. Did I miss some important info, if he is going to use a np205 with the 6l80 than he will need to call on Advance adapters since they make the 6l80 to np205 adaptor only 700 beans. why spend the beans if he already has the transfer case that can be used with the 6l80e

[mongocanfly]

Big difference..
1972 k20 transfer case is pass side drop
2009 GMC transfer case is driver side drop...

[YoungPup1977]

Originally Posted by mongocanfly

Big difference..
1972 k20 transfer case is pass side drop
2009 GMC transfer case is driver side drop...

So you already know what he is going to do. Buy a adapter kit from advance adapters. You really know your stuff. Would he ever consider using the donor frame ? Just wondering.

[mongocanfly]

Another option would be "try" and find a 4l80/np241 from early 90s square body burb...
The np241 is pass side drop hooked to a 4l80
Or find a 4x4 700r4...to put behind the LS
Frame swaps are difficult to say the least...unless youve got a nicely equipped shop...and then the 72 body wont fit the 09 frame very good

[CRMW]

Finally getting a little closer to this swap. I've seen lots of write ups on swaps with the 4L80 and/or 6L80 swaps but I'm having trouble finding something on the 5.3 & 6l80 swap. Can anyone point me in the right direction?

Chris

[rowdy11]

I’m swapping an L96 with a native 6L90. Ive already adapted my transfer case and will be making the final adjustments to the 6L90 tail housing in about 4 weeks. You’re welcome to follow along. The mechanics of the trans are pretty straightforward but the bigger challenge will be adapting 6 speeds to a column shifter . I’m still deciding if I want to rig up a paddle shift.

Best of luck and should you choose to go that route, the advance adapters kit is well made.

[Mrgeez90]

Originally Posted by rowdy11

I’m swapping an L96 with a native 6L90. Ive already adapted my transfer case and will be making the final adjustments to the 6L90 tail housing in about 4 weeks. You’re welcome to follow along. The mechanics of the trans are pretty straightforward but the bigger challenge will be adapting 6 speeds to a column shifter . I’m still deciding if I want to rig up a paddle shift.

Best of luck and should you choose to go that route, the advance adapters kit is well made.

What were some issues you ran into with this setup? I’m looking at doing this setup in my 72 K5.

[rowdy11]

I had to trim the output shaft on the transmission (and the input from the advance adapters kit on the NP205) to ensure they fit. Some 6L80 and 90's have super long output shafts. I will also have to custom fabricate a shift mount on the transmission for the transfer case shifter, since it doesn't have a bolt up like the TH350 had.

I also did some fabrication to keep my original trans cross member and mounting location. You can see the pictures on my thread.

It came together well...just takes some work.