If you have a die grinder with a cylindrical burr on it you can oval out those holes. Before you do that, trace the holes on a piece of cardboard and make sure they are exactly the same, not worn to the shape you see now. I am not entirely convinced the holes were oval originally but don't take my opinion on it. Also, if you have a chassis punch like electricians use you can use it to take a bite out of the existing hole and get it closer to the shape you might want.
As far as tanks go the second one looks strange to me.. Who supplies that one and what is it supposed to fit? I have quite a few original tanks that leak badly, and if I have time today and can find a good one I will see if I can find one that hasn't been patched up to take a pic of. The Omix tank may fit their tub, I do know some have had problems with it fitting a stock tub.