If anyone here is interested...
After a fair amount of discussion and advice from the crew here at RCM, I got my plastic Jeep fuel tank fixed.
After removing the tank, and clearing it of nasty flamable vapors (several fill/dump/repeat cycles with water over several days and a several week bake in the hot sun, openings up, of course) and much pondering, I decided I had nothing to loose, as the tank was no good anyhow. So I did a quick test with a junk screwdriver. I heated the blade up with a small torch and touched the hot metal to a little nub sticking out of the top of the tank. The plastic melted and I found that I could reshape it, and it rehardened when it cooled. Neat-O, thermoplastic!
So I applied the hot blade treatment to the 2 small cuts in the tank, melting the stuff as deeply as I dared without softening it all of the way through, and sort of smoothed over the damage and fused it until it looked closed up. Did not come out too badly. Beacuse of the way this tank sits in the vehicle, and the way the skid plates and whatnot go in, it is a hugh pain-in-the-neck to install and remove. So I went the extra mile and tried to seal it with some sort of adhesive. JB weld did not adhere to the plastic very well, and the locktite fuel tank repair stuff the someone suggested specifically says not for plastic tanks. So I tried one of the first suggestions I got from you guys and tried SEAL-All. This stuff worked great! it hardened to a nice skin that I built up in several layers, and I spread over maybe twice the area of the original patch, just for good measure. The solvent in it seemed to partially disolve the plastic of the tank, so the stuff fused nicely to the surface. I did "bench testing" on the repair for a few days while we did some other work to the truck, then put it in.
Been in the truck a couple of weeks now, and seems to be a solid repair.
Thanks for all of the ideas and help. It can be done! Another small victory for the cheapskates.