Well, I don't think the 74 quart would be too big, but I think the 60 quart would be perfectly fine and, like you said, easier to control temps and humidity. Ultimately it's your decision though.

Also, I'm not sure I can completely visualize what you're saying with the flexwatt (and I don't have any experience with it myself), but I'd say if you get the 60 quart and turn it the way you were describing so that it would cover 11 of the 34 inches, that would be great since that would be pretty much a third of the bottom. And then, like Vypyrz said, you could always trim it down to get the temps you need. The entire flexwatt doesn't need to be in contact with the bottom of the tub, if that's what you're asking... just however you can get it to cover more or less 1/3 of the bottom.