4l80E/NP205 (Yes, another one)

[ZacharyDouglas1992]

Alright well this is my first time posting anything on here (at least on purpose) so go easy on me haha.

I am seeking any insight as far as how to go about this swap. I've seen 500 million of these online, but rarely does the poster give enough specifics as to the vehicle/parts in possession. I am going to try to be as specific as possible and feel free to ask me anything I have left out.

The truck is a 1983 GMC Sierra Classic crew cab K3500.
454/TH400/NP205-DANA 70HD 4.56. Now you can see why the overdrive is attractive. with 31" tires I am 3000rpm+ at 65.

I just purchased a 454/4l80E/unknown transfer case combo. (I havent picked it up yet and I didn't crawl under the vehicle so it won't remain unknown) However this is from a 1991 Driver side drop, and i would like to stick with the gear driven, bulletproof 205. Honestly the price, to me, was worth the serpentine set up from the engine, everything else is gravy.

I will be rebuilding the 4L80E, or having it redone, and updating it so that the aftermarket tranny controller can be used.

The real question is the best way to go about mating the 4L80 to the 205. I would think the best way to go about that would be to have my current TH400 adapter reworked. Machine work on the indexing ring? If anyone has the specifics on this, or knows somewhere I can send it to have it done, I am all ears. If anyone here has done it, and is willing to do my adapter, name a price and maybe we can work something out. I think going that route involves shaving a bit off the 4L80 tailshaft as well? Again, specifics would be great.

I am especially interested in the input of those who have used the TH400 adapter method before, but I am open to other Ideas.

Also, depending on how this goes, I will be selling the TH400. I just had it rebuild 4000 miles ago. still has a warantee through december. had an HD shift kit installed. It shifts fantastic, I just can't live without overdrive with that gearing. Probably for around 600, about half what I've got in it. And im in central CA if anyones truly interested. On the flip side if this seems to overwhelming I will sell the 4l80 for 150 as a core. Was in good condition when parked, but truck sat for 12 years.

*notes
The truck is one of 5 vehicles and far from a daily driver, so turnaround time isnt a huge concern.

I just had the rearend completely rebuilt, and I was told 4.56 was the minimum I could do in that diff, so changing the gearing is not a route I'm going to take.