Gooseneck fifth wheel and a flat bed. Help!

Lots of blank spaces on what your truck is and what was done to it.
The trucks gvwr won't change nor will you get state certified vehicle modifier to change the trucks gvwr.

Those 3.42 gears are not a good gear for towing heavy trailers and will limit you on the 5th wheel trailer size/weight. As far as dot/insurance is concerned its still a 3/4 ton truck.

No doubt a built for towing 350 and a built for towing Th400 tranny with a big aux cooler won't have any issues.

As another poster says you won't have tail wagging the dog with a 5th wheel trailer like a bumper pull. Apples vs oranges.

I would stick with a 10k gvw trailer...mostly because of the tall 3.42 gears.
Now go with some 4.10 gears/suspension/brakes/the whole works from a late model one ton srw truck can give you another 2k-4k gross weight trailer.

Well the upshot of that is that the truck has a 4spd instead of the turbo 400. The Biggest question I have is that fact of what weight campers I should be looking at. I am not scared to drag my powerstroke out of retirement if needed but I would prefer not to. My wife is the one that wants a fifth wheel and I would settle for what ever. Does not make a difference to me s long as it has and outdoor kitchen. Yes a I know that is a sad request but a man has to have his say.

The 4-speed doesn't help. 3.42 gears are a problem, limiting you to ~5000lbs of towing, if I'm not mistaken, and then not very comfortable towing. Lots of time down in 3rd (really 2nd because of the granny low).

On an old school truck like that you really need 4.10 or even 4.30 gears. This will get the RPMs of the engine up so that you can tow. Even then you're probably only looking at 8000lbs if you want any comfort level at all. That's a pretty small 5th wheel.

With newer trucks the gearing isn't so much of an issue because you have more gears to choose from, and more evenly-spaced. On your truck there's such a gap between top gear and the next one down...

Sounds to me like I need to scrap the idea of using the K20 and drag the powerstroke out. From my measurement at a local RV sales lot, I need to stay under 30' regardless due to my shed. If I get over 30', not only will I be buying a camper I will be adding on to my shed also. That being the case it is a 1997 OBS ext cab long bed f-250 with the heavy service spring package and the 7.3 turbo diesel. It is bone stock, nothing ever done to it performance wise. Still has the stock intake and exhaust. I know I have had a combined weight of trailer and load at 14K behind it and I know that I don't want to do that EVER again. Just a little piece of advise and I am sure I am preaching to the choir on this one, do not let family talk you into hauling stuff for them. There was a lot of lost religion when I got home.

use the truck. Add an ht502 crate motor and it would be a perfect rig. My 76 dually works well with 18500 behind it

Nice job! When most people say they converted a 3/4 ton to a one-ton all they did was put more leaf springs in it. You actually put EVERYTHING in (expect maybe the hydro boost brake system?) and anyone who really knows trucks knows you DID substantially increase your GVWR. The hard parts are all there. There will always be the guy who says if the factory didn't do it, it didn't happen.

I've done all that stuff a few times myself. One of my old trucks was a Ford that carried a camper and towed a boat with plenty of power (a very "built" '68 428) and it was also a 4-speed and 3.50 gears, similar to your truck. That truck was right in the happy zone empty but the gearing sucked under a heavy load. The jump from 3rd to 4th was just too big and I had a hard time holding 4th up hills under a big load... and I had a LOT of horsepower. Gas motors just don't make their power at low enough revs to use tall gears with heavy weight. I swapped the entire chassis of that truck for another using 4.10 gears and it was a much better heavy load rig. It used to spank all the diesels about 17 years ago. (Not now!!!)
Your rig sounds perfectly up to the job, all except those tall gears.
The transmission will handle it, that K20 would have come with the same SM465 my '86 K3500 dually has.

I've had a 14,000 pound load (camper plus trailer) on my '86 GMC many times over many mountains without issue. Sure it's a little slow, but it gets there. And unlike a more powerful modern truck, if something does go wrong, you can open the hood, see it, and fix it easily and cheaply! But mine is a 454 with 4.10s.

I have a friend who carries a very heavy 11.5 foot camper and tows an enclosed trailer full of snowmobiles over the mountains with a '77 Chevy 4x4 crewcab dually with a 350 and 4-speed but he has 4.10s. And he's looking at doing a 383 stroker.

If I had to choose between the truck with the 350 and 3.42 gears and the '97 7.3 diesel, I do think I'd use the diesel.