Deep sump "Milodon" type oil pans, need special gasket?

There are basically two types of oil pan gaskets available. One is the standard 4 piece that has cork along the rails and rubber over the crank ends. The other is a one piece rubber. In either case, the are both the same thickness. I prefer the 4 piece, never had any leaks. I also use a stud kit to simplify alignment.

My advice... don't skimp on the pans. The last thing you need is to discover an oil leak at the pan after you spend weeks building the engine and installing it.

I just purchased a Canton 7-Quart from http://www.adperformance.com/ for $180. I've been shopping for months and they have the absolute lowest price I could find.

$0.02

Sounds like a poor quality oil pan.

Couple things you might consider...

Do you have the correct pump pickup? Is in conflict with something in the pan, maybe a baffle?
Do you have a high volume pump? If so, some pans are more picky than others if the pickup will touch the bottom or not. I know my Moroso pan and pickup specifically stated in the paperwork that it would not work with a high volume pump because the pickup sits about .25" lower than stock.
I used a high volume pump anyway. I carefully checked everything, even though it was very tight, it fit. I guess I was just lucky, because some folks had that problem for Moroso to warn against it.

Next issue:
Did you tighten the middle bolts first and work your way out from center of the pan?
There is a torque sequence to the oil pan too in case you didn't know and started from one end and went to the other.
It doesn't mean the pan is bad, just that you crushed the rear gasket unevenly and pushed alot of clearance to the other side of the pan.
Even a warped pan will seal fine if you use the torque pattern.
If the pan is currently warped, and you did not use the pattern or didn't check pickup clearance, then it is very possible that you warped it by trying to install it. So don't jump too fast to blame the manufacturer. Like I said, it should seal fine when properly installed.
Good luck
Dave

Edit: A couple pointers that work for any kind of pan... tranny, oil, valve covers, etc...
If the pan has been used before, the bolt holes tend to bow up towards the gasket surface. Any time you see that, take a hammer and lightly pound the raised hole back down. This helps seal the gasket more evenly.
If you did not use the correct torque pattern, check the gasket very carefully, you could have depressed it in spots and it may not be flat enough to work again. Time for a new gasket.
The depressions that run in between the bolt holes along the edge of some pans are great places to put a very light amount of RTV gasket sealer. I put mine only on the pan side, not the block, and I use a very small amount. Concentrate the RTV in the depressions, place your gasket firmly and straight, let it dry for a few minutes so the gasket doesn't shift. Now install the pan.