1980 Chevrolet Truck cross threading

It is hard to say if it will leak. It all depends how bad you crossed up the threads. It sounds like you got it back tight so hopefully it won't. Forma a gasket will help but transmission fluid is under pressure and is pretty thin in viscosity. The only way to know is to run the car and check it after you have had it up to operating temperature and driven it a few miles. It is a tapered fitting so it is usually a good seal when you can get it tight. I would recommend, if it leaks, getting a pipe thread tap that matches, not a standard thread as if it is a hard line fitting or like a brake line it is a pipe thread and tapered as opposed to a standard straight thread. So you will have to find a pipe thread tap which is not easy to find. If you have a Harbor Freight Tools near you they are a good bet. Of course try Home Depot but they may not have them. Use the tap to clean the threads out and use lubricant when you do it like WD-40. When you turn it turn it backwards every full turn or so, you will get a feel for when it starts to resist turning, and turn the tap backwards to clean the threads out. Wipe the metal out with a twisted up end of a paper towel and then spray the threads out clean. The fitting on the pipe is probably harder than the aluminum that you are threading into so you can just spray it and wipe it off but if you can get a pipe thread die, it would not hurt to clean them up. This will give you the tightest fit possible. There is a repair that allows you to drill out the hole, tap it and insert a steel thread, but it does not come in pipe thread as far as I know.
When you put it back together use Teflon tape or Teflon thread sealant. That will be your best bet.
It is not uncommon to cross threads. If it weren't I would not have a full set of taps and dies!

Doesn't sound like junk to me. If you look at the cars the techs on our team has they are all almost over 100,000 by far. I have a 1990 Integra with a 1998 JDM GSR engine swap and every bushing and suspension part replaced, gauges galore and custom interior and I have about 2500 hours into all myself. That includes when I had to work on it all night to get to work in it the next day. It is fully restored like new and it took me years and tears and everyone said I should get rid of it. Well, its a VTEC now with over 200HP and it weighs about 2000Lbs. So it gets up and goes especially when the VTEC kicks in. I have beat alot of cars except for an S2000. It sounds like your truck will be pretty fast. If your tranny cooler leaks a little don't freak out. You can always replace the pipe fitting and get an aftermarket radiator that will cool better for $200 or less.
I think you will be OK. One other thing you can do is put JB Weld around it if it leaks becasue its brittle and you can usually pop it off with a chisel. But its a gamble.
Good luck and Its no peace of junk. I would give you my website so you can see my cars but we can't do that any more. Just put carrepairlinks together with a www and a com and check it out. I hope this doesn't get me in trouble but you'll see my 340 Dart swinger and it was the sleeper.
Take care
Come back anytime you need us you will fiind our techs are very helpful.
Dr. C