86 Iroc Z Fuel tank over pressurized, fuel pump whines in tank.?

I have an 86 Iroc Z and I have already replaced my fuel pump because it completely burned out. The new Bosch fuel pump is now whining in the tank because the tank is over pressurized. Sounds like I have a dying owl in my tank. Happens more frequently when its hot. So as long as im not driving very far for too long then everything is fine. If its a hot day then after a few miles the pump starts to whine and I have to pull over and open the cap, every few miles. I asked around for the vent and not even the dealer could get me one. Nappa had no Idea what I was talking about untill i had the guy crawl underneath the car to show him. I was also told by some part stores the there is no vented cap for my car, but nappa said that there was. Im confused and I would like to be able to drive over 50 a a day. other than that the car runs great. What has been done to solve this tank vent? should I close the line and get a vented cap, or just take the little plastic vent off and leave it open???
86 Iroc Z Fuel tank over pressurized, fuel pump whines in tank.?
Before you do anything...check with your local DMV and see if vented fuel caps will pass inspection where you live. But if you care, a vented gas cap is really, really bad for the environment since you end up venting unburned gas into the air.