Need help figuring weight capacity

Weigh your TV rear axle hooded up and fully loaded. If your weight is less than the tires max weight rating X2 you will be fine.

You have the truck and trailer. How does the old IDI engine handle the load ??

Is your truck squatting too much when the trailer is hooked up ??

Those old trailers were pulled by those old trucks every day back in that era. We didn't have forums back then with weight police types telling others it can't do it without any actual weight numbers.
Sure they were a bit slow compared to todays trucks but everything was slow back then. Even the king kong 454/460 big block gassers which were the biggest engines (235-250 HP) in our LDT's of that era.

Just keep in mind the truck is 20 years old and from another era so don't try keeping up with the flow.

Weigh your truck....full of everything you would have in it to go camping (you, passengers, fuel etc)
Subtract that number from your truck GVWR...8800#

That 5th wheel will have a pin weight around 2100# (if truck weighs 6700# you would be at GVWR)

Heck....you have the truck, you have the 5vr.....go get them weighed (camp ready) then you will know how much you are over/under etc.

Would I be better off switching to my 1997 Chevy K3500 Crew Cab 4x4 with a 5.7 L V8 gas engine? I know it is dangerous to surpass your GVWR rating.

Think you will be over your GVWR, but the 7.3 will do the job, and you will likely do just fine.
The decision to tow over your GVWR is yours. I tow an 11,000# 5er with my 01 Cummins, and the same GVWR.

Have you read RVing FAQ above link. Look under #3 category for the formula you are looking for.