Honestly this whole thing is odd. From 84-90 it is very common to have a 700R4 in a 3/4 ton. It is not very common to see a 6 lug 2500, especially in 88. Pics would save A LOT of this back and forth. First half of the VIN would be super helpful also(without needing to worry about any privacy issues). Picture of the engine bay. Picture of the rear axle from the back showing the cover. Picture of the transmission pan. Done.
7500lbs is a lot of weight. You'll know it's back there for sure. If it's a nice flat road where you don't need to hit any highways.. you'll probably be ok. But it sounds like you aren't very experienced and you have no history in knowing what kind of condition the equipment is in... it's pretty sketchy.
I love the assumption that I'm not experienced in towing anything. I have actually towed quite a lot, I'm not used to however towing a 5th wheel trailer. All the rv's, trailers, and toy haulers I have towed were bumper/frame hitch. All I've really needed to do in these cases for the most part is make sure I have the right hitch and truck that could support it. However I know 5th wheele trailers have different weight distribution, handling abilities and things of this nature. Only reason I'm willing to take on the beast that I am, is from the experience of driving all the other trailers I have towed in the past.
But after going over info with my brother, who has experience in towing things like this and his input. I realized that he was probably right, so I wanted to get others input the label gt4 which is rated at 6-8k pounds. So thank you all who saved me from blowing up my truck, I appreciate all the in put. I have learned though that since I have the 14 bolt pumpkin, I can upgrade the gears from I think like 37 to 41 or something. Which would take it from a 6-8k tow rating to 10k tow rating, ofcourse it will take more then just that one upgrade. There is a possibility to give her more, tow capacity then she is rated at now. But I will take everyone's advice and probably restore her as much as my possibility. Then find an other person who respects the classics to sell her to and finish her restore process.
As for me not posting pics like everyone other person keeps telling me to. It's not that I wouldn't like to. My phone dosnt like doing that type of thing and when I can get it to most of the time they upload super burry for some reason.