Fuel Problems, gas in vapor line, leaks, scary!

[PHOENIX]

Well awhile back I posted about my vapor canister being full of gas and not being sure where it came from. I have been cruising around with an empty water bottle hooked to the vapor line, which vents the gas tank. Its been getting hotter and hotter here. Well today was the day.

I am on my way to lunch (about 2 miles from work) and start to smell gas. Last time I smelled gas like this the truck died, the carb was flooded. I had the carb rebuilt and though maybe the problem was solved. Maybe the needle and seat was stuck and caused the carb to floow and fill up the canister. Today with the water bottle attached to the line I determined that the gas was coming from the gas tank. Not sure exactly what is happening and thats what I need help on.

My truck now has headers, new cat, and a single 3" exhuast running down the drivers side, over the axle and out behind the tire. The system is very similar to the stock system, but with headers, different y-pipe and 3" exhuast from there out.

My first guess was that the gas tank is getting hot and the gas inside is expanding. It expanded enough for the gas to go into the vapor vent line and all the way to the canister up front. The 24oz water bottle was full of gas and it was leaking out the top and onto the frame and ground. I dumped the gas out and went in to get food. Came back out and the bottle was still empty. I drive back to work and go inside to eat.

After a couple hours I notice a spot under the truck. Its gas leaking from the the tank switch valve. It looked like it was leaking around the clamps, but I couldn't tell for sure and was not about to lay down on the hot asphualt. The valve is broken and stuck on the drivers side so thats the only tank I use. I estimate that it leaked somewhere around 12oz at that location. I checked the bottle again and it was around 60% full (from a 2 mile drive and a couple hour sit!). I dumped the bottle again and put back. I then wondered how much pressure must be in the tank to cause a leak at the switch valve. So I slowly opened the gas tank cap. There was obviously a higher amount of pressure then normal. I put the cap back on and went back to work.

About 30 minutes after that it was quitting time. I came out and checked the bottle again, still empty. Looked at the switch valve and noticed that is still had a very slow drip, much slower then before. I removed the tank cap again and more pressure was released, still more then normally. I decided to leave the cap off for the 6.5 mile(10-15 min.) drive home.

I arrive home and check the bottle, its about 15% full. The switch valve leak was even slower. When I took the gas cap off there wasn't much release.

I assumed that the gas cap is not supposed to be vented, thats what the canister system is for. Am I wrong? Is the gas tank cap supposed to vent the tank?

What do you guys think my problem is?
If it is a gas expanding problem then I'll have to make a heat shield to protect the gas tank from the exhuast pipe. The shield would be similar to the cat shield.

Please help me out guys, thanks!