I'm sure you won't have any problems with too much heat I'm sure the fuel tank under your pickup catches more heat than one that will be inside your bed

 

I agree you should be fine. My buddys dad has an aux tank in the bed with a single stack running right up along beside it. About 1-2 in gap. He doesnt have a ypipe of coarse. You have duals? Do you plan on the tank being above the y-pipe?

 

I think I'm going to a single stack and cut a square out the tank for it to come up threw. I have a 100 gallon tank I built to fit a shortbed. I was going to cut angles so it would fit behind the stacks and on the fenderwells but I got to measuring and my fifth wheel ball would set right against it. I think I'll wrap the exhaust to the top of the tank. I'm thinking a inch of clearance around the exhaust ought to be enough shouldn't it?