Correction: I'm not going to modify a tank to fit my car! Nothing is anywhere near the correct size. Mines only 22" from front to rear (of car), 39" side to side, 7" deep in front, 4" deep in rear. Closest I can find if 24.5" front to back, otherwise it would fit. Since I can make duct work, I'm going to build my own tank!
I'm not sure how I'm going to seal the seams yet. They will be folded pocket seams, which don't have a lot of strength, but I'll spot weld every four to six inches just to be safe. I can put epoxy in the pockets before crimping, solder the seams after assembling, and/or use a tank sealer. The eopxy I think will be the best -- I know it will seal. Solder may not penetrate the pockets enough to seal, though I'm making it like an open tank with a top. So I could solder the lower seams from the inside (the ones that are most important), which would be more resistant to leaks. Solder joints can crack though, epoxy is much less likely too. I could also use something like the Hirsch Auto sealer and forget solder and epoxy, or solder/eopxy AND use the sealer. I'm going to use the rivets no matter which way I go. It will take quite a hit to burst a seam, but the sealer could break loose easier than epoxy or solder.
Oh well, I'll have time to think about it! I'm leaving a 5" spot along the front the width of the tank 3/4" lower than the top of the tank. That way I can mount the pickup/sending unit on a flat on top, and the extra 3/4" will be a nice expansion area. I thought about slanting this area a bit more than 3/4" (maybe a whole inch!) but the edge of the sending unit mount would be submerged for a longer period when the tank is full. So I opted to drop the gallon of capacity and leave the expansion area instead.
I'm also thinking about permanently installing the pickup tube instead of mounting it on the sending unit. I could then cut a large access panel on the top (above the expansion area) to allow changing the sock on the end of the pickup. Or I could install the tube and cut the access later if the sock ever needs replacing. The return fuel (this is for an EFI engine) and vent openings will be in the expansion area also. I'll likely install a baffle in the center, and one along the front edge about an inch tall to keep fuel near the pickup when low. Looks like I can make the whole thing from a single 36"x96" sheet of steel, though I may have to find a scrap for the baffle.