This has been nagging me on my '92 for a long time now. Near as I can figure, the problem is the fuel and its vapor heats up and expands so fast the check valve closes when the fuel sloshes, and then the pressure holds it shut. I have done all of the following, to no avail:
Wrapped the tailpipes around the tank with insulating pipe wrap
Relocated the lines above the tailpipes a few inches further away
Wrapped the lines in insulating wrap
Attached sheet aluminum shielding between the tank and the tailpipes
Reversed the spring in the check valve on top of the tank
I now live 10 minutes from Bondurant's facility so I stopped by to ask thier shop chief what they used to do about it when they ran Mustangs. He suggested running the biggest, longest vent line I could muster, and attaching it so it vented out the center under the bumper. He warned me that even then, it would likely piss out the vent, but at least then it wouldn't be next to hot tailpipes.
So I got a rubber stopper and a 90-degree 3/8 elbow and plugged it in the top of the tank I attached something like 10 feet of 3/8 fuel line, coiled on top of the tank and then routed down and secured near the center bottom of the bumper cover. Then I turned the end up vertical and installed a check valve from pegasus Pegasus - Fuel Vent Check Valve, In-Line for 3/8 inch Hose
because I just didn't feel right having the vent line unchecked in the event of a rollover.
It works fine as long as temps don't get too high, but when it was 104 at PIR with a track temp of over 135, by the time I was near the end of a 20-minute session, the tank was pressurizing and pissing out the cap again.
Another thing that seems to have helped is I clamped a stand-off to the passenger side tailpipe to keep the pipe from getting shoved right up next to the tank by the tire rubbing against it. Now it's got a minimum 1/2-inch clearance at all times. I'll know if it worked when I get it back to a really warm track day.
Another example: https://forums.corral.net/forums/show....php?t=1094636