No, Chuck, I don't think it will end up looking like a hot dog. But it will have nearly 15 tons of force pushing that flat end out and it will dish (I don't know how much) and that will put forces on the welds that they aren't designed to take. They wouldn't need to fail explosively to ruin your whole week. A leak is as bad as a flood in January. Rereading your post I realize you don't need to shorten the tank. Cleaning the inside of the tank is a great idea and I would like to pressure wash mine as well before I use them. Looks like Locust Loco cut a hole in the end and used a long wand guessing from the photo. Being you're experienced with heavy welding why not just remove and replace the round head. With spherical ends these things will tip up within vertical clearance about their overall length. With a 4ft. square end it might not clear your ceiling before it gets vertical. The shells on some modern 500 gallon tanks are about 1/4" thick if they are about 36" dia. The heads are 1/32" or so thinner. Old ones are made of thicker plate, I think; at least mine are (40 years old). I'm limited to available equipment. My tractor won't quite pick them up vertically and I only have gas welding gear so I'm not confident to do it myself. I have access to the welding shop and welder at work (he welds the housings of some the biggest gear boxes in the world). I envy you guys with real tools. How do you plan to add a 2nd tank later? Drain the system and weld them together with some kind of pipe or just threaded pipe fittings? I keep tossing back and forth on this one. I have two 500gal tanks now and am expecting that's all I need.