Most decent radiator shops can clean the tank for you. You'll have to drain the tank before taking it to them. Then simply find a welding shop and get after it. I've even had the radiator shop weld it in.
Like this? I made this sump out of scrap 18 gauge sheetmetal and welded 3/4" pipe couplers (cut in half). Make sure to use black iron couplers or other threaded steel. I used galvanized ones and had to grind and grind and grind to get the zinc off them so they would weld half decent. I knew better - just had a brain fart. I did this in preparation for bigger engine which I am just now in the process on doing. (one for fuel, one for drain, and one for possible nitrous) If you want to weld the tank quickly - stop-up all the holes (fuel fill, vents, etc) and fill it over the top with water. Then you can use cut off wheel to make your cuts and weld the same day. If you have more time - drill and use tin snips to cut your hole (no sparks). Then use compressed air, hot sun, and about a week to evaporate all the vapors out before welding. Cleaver
