Here's what I would do. You say you have some scrap left over from the same tank, so it's for sure cast? (If the tank is not made out of cast, I wouldn't bother fixing it) Anyhow, cut your patch, 8" square is fine for a normal sized bukhead hole (around 2" or less, right?) and mark around it where it's going to go on the tank bottom. You could go smaller, maybe 5" square? You need to wet sand that part, start with 320 wet/dry paper, go to 600, then if you're looking for more stuff to do with your time you could use a couple of grades of micromesh, but it's not necessary. Just make sure all the crud is off. I'd put some masking tape around the area, leaving 1/4" or so all the way around. It's not really important if the whole thing is going to be covered up, but you might as well.
Then you should round off the corners of your patch, and if you have a quarter round bit, round off the whole top edge. The reason for this is to not leave any sharp corners exposed; it's possible you might catch one on a rock someday and crack the patch...unlikely, but why take a chance? If you don't have the round over bit, you could do the same thing with a sanding block, it'll just take some time.
For a good strong bond, I'd use weldon #40; it's a two part polimerizing cement. It's MUCH stronger than weldon #16. The #16 might work, but the #40 will definitely do the job and last forever. It's a little hard to find, you might try plastix (I think) in New Mexico. Mix up some of that stuff, and spread it around your area on the tank, and just press on your patch, making sure there's enough of the stuff to squeeze out onto the masking tape. You'll want to pull the tape before the glue hardens. You won't need to clamp it, but you could put a little weight on it if you want, maybe a quart jar of water or something like that. Let it dry for a day and you're done. I know this way of doing it seems a little tedious, but it will really work and you won't have anything to worry about.
