I've got a 12oz tank in my .46 AX powered Mustang. I can get about 17 minutes or so WOT with that, but at that speed, 10 or 12 is enough for me. Your .40 should be a bit less thirsty than my 46AX.

Good rule of thumb. Put as big a tank as you can fit in there and still have some foam around it.

I like the mouldable sullivan tanks, you can make'em fit with a heat gun and a few minutes. Very handy for maximizing fuel range.

J