Dear Reza,
thanks for the comment ,
Feul Tank has many ways to let the high vapor pressure go out in example : Cap connection is not too tight - the pipe that carrys the fuel to the pump and carburator - the pipe that returns the extra fuel from carburator back to the tank , ... .
so there would not be very more pressure than 1 atmospheres .if i remember correct the pressure of the mixture that explodes in the engine cylinders is about (air)7:1(fuel) .
but in the tank it would be (air)1:2(fuel).

and sometime ago i read a article about DIY EFI (Electronic Fuel Injection) here and i searched the net and found someone get 50 HPs more with his 71 camaro ! with implementing EFI.so i am about to try it soon.

Best Regrads :
Farshid J.H.