If they are custom builders and THEY screwed up, they should not be making you wait 8 weeks for a new tank, it doesn't take anywhere near that long to make a tank. They are putting you at the end of the list again. 2 weeks, maybe 3 tops, and that's still making you wait too long. You're the customer, you paid them, they screwed up. Not saying make them dance, but get what you paid for. You're going to live with this tank for a few years at least, think about all the times you'll look at your "settled upon" tank and think, I just should have waited.
They need to put you at the front of the list, knock it out, and get you taken care of. I've been in customer service a LONG time, going on 25 years, and the customer in front of you who's given you money and you made a promise to comes first. Period.