Early GMT400's with the 6.X's could come with a 700R4. They weren't all electronic transmissions.
I'm going to be the downer here: swapping a 6.X in is the wrong way to go. Swapping one in is unique, but only because they are not that good of a choice and almost everyone "in the know" would avoid it. Add in the fact that once it's in you have a powerplant option that was available factory and you've just lost anything "Unique" about it. In fact: most who look under your hood will see a 6.X and think less of the vehicle. It's the stigma/bad reputation the engine carries with it and it's a "well earned" reputation.....
I would seriously recommend you abandon the 6.X idea and stop dropping good cash on it. I had to learn this the hard/expensive way and would prefer to spare you that experience.
I'm not some random 6.X dissenter either. I've popped both 506 and 599 blocks in just regular use and I am currently running a 6500 Optimizer.
The GM/DD designs are just not good castings, at all. 506 being the worst. The main webs crack at the cap bolts and cap registers, the cylinders are known to circumferential crack, the heads crack in the valve lands, starter mounting pads crack off, harmonic dampers that come apart internally, head gaskets give when the boost is dialed up (maybe not the first day, or a month later, but they will go they're just not supported enough), injector pumps are not hardened for ULSD (LSD only) and Gawd help you if you go with an electronic diesel version which exhibit all manner of electrical gremlins. And there's more that I won't even go into.
Causes are everything from high mileage, poor parts, poor initial design and OE manufacturing problems/mistakes.
These are all fixable things, but it's going to require lots of $$$$. Block and head problems are virtually eliminated with a new GEP assembly (7,000), injector pumps are available with revised ceramic parts (1200-1500), revised and relocated pump driver module (approx 500), harmonic dampers replaced with fluidampr product (500) and on and on.....
All the rebuilders I know won't even bother cracking open a 6.5 for rebuild, it's toss it out and go new. Often with a "boat" casting. You find about 1 good used block in 10. Heads are just as bad. I've been to scrap yards that won't even sell you a 6.5 as a used replacement, too many come backs. The only thing I've seen rebuilder and resellers consider worth while is a NA 6.2/6.5. They live a low stress enough life to have a chance of not being junk on a pull/rebuild.
I would really suggest staying gas and doing a 383 or 454 over the GM 6.x engines, especially if you're just building a "fun" truck. A 6.X will suck the fun out of it in pretty short order....
If you want unique and diesel, go 4BT/6BT or any of the HINO or isuzu diesels.
When my last 599 popped, I was stuck in the middle of a long trip with 8000lbs in tow. Circumstances dictated that I replace it. I went with a 6500 optimizer and it cost me damned near 10 grand when it was all said and done (parts plus labor, I had no shop available). Just an FYI; when we cracked open the 599, it was riddled with cracks in all the usually places. It failed because a piston crown cracked, but it was well on it's way to a classical 6.x failure anyways. In all fairness, it did have 300,000 kms on it though....
Given my druthers, I would have had a 6BT swapped in if I had of been another 300 miles in one direction (closer to a shop that specializes in 6BT swaps) and have simply dealt with replacing unit bearings and front end components every few years.
That being said, the Optimizer is holding up well. I've squeezed 271 RWHP out of it on a chassis dyno. But that's just barely getting in to the stock duramax range.
I've since dialed it back down for longevity. Even cracking 200 HP is an accomplishment on a 6.X, they just a re not powerhouses. No, it wasn't easy or cheap. I've since seen the light and have pretty much stopped trying with it. There's an inter-cooler project on the horizon for it and then it's run it until it dies (hopefully after another ten years so i can get some return on what I've spent on it). The truck I keep working on, the engine is now mostly an orphan as far as I'm concerned....
Personally, if I would have had the time, a Duramax would be sitting where the optimizer now resides. There's even a couple swap manuals out there on it. But I was stuck in both location and by time constraints so the only thing that worked was "replace".
The one application I would say a 6.X is good for though is a diesel off road crawler type application. It drops in like a 350 into something like a jeep chassis, has excellent low power delivery characteristics and it a fuel sipper on low speed trails. Off camber fuel delivery is also a bonus as it's unaffected by lean angles like a carburetor would be.
The only other thing I would recommend for someone buying a 6.X is to consider it a driver until it starts causing problems that take more than $500 at a time to fix. At that point, it's time to thank it for the service it's given and send it on it's way down the road. When You start getting into replacing heads, blocks, turbo's, etc, it's just going to be a money pit....
Sorry to be a downer, but thems the facts as I see/experienced them.....