Here's a few hints.
1: Best to replace the fuel tank with new if you can get it imported. You'll need to save the filler neck as replacements don't come with one. Ther 1979 - 1980 Firebird fillers necks were longer then the previous years due to the redesign of the tail-lights and bumper. Which leads to idea # 2.
2: After running the gas tank 'dry' and getting all the gas out, take it a shop that restores radiators or find an old (relative speaking as some folks think anything post war is too new, joke) auto restoration shop. Have them 'boil out' the tank which will remove the rust and expose all holes. They can then weld the tank back together. That is if they agree to it. About the filler neck, had a 79 that leaked at the top of the joint between the tank and neck. The reason is pressure of the gas pump handle resting on the neck during refuling cracked it. Found out when it was replaced with a 'new' old tank out of a 78.
3: While ordering the tank, order new hold down straps. Safety first. While the tank is out, use a POR 15 to paint over the trunck floor. This stuff sticks to rust and metal like glue to paper. Its made in New Jersey (USA) but maybe sold in the UK, do a Googly.
Now about the gas tank pressure. Two things. One, the gas cap vent could have failed. Get a new cap. If your need OEM, there are lots web sites supplying GM parts. Its a common part. Else, maybe your local auto parts store ight, agian don't know the easy of parts access over there.
Two, the charcoal gas vapor recover system canister needs replacement. First replace the gas cap, then all the rubber hoses to the canister. If that does not cut down on the pressure, then replace the canister. That verywell will be a OEM part. What happens is the canister captures gas vapors from the tank. Then, when the motor is running, the canister is purged by the vacume line and burned in normal operations. What is happening is the vapors are not making it to the canister.
Check all the lines for pinches. After all this it still happening, it could be a rusty cloged line from the tank to the canister. The fuel system is simple, tank, pump, carborator, vapor emission system (canister). Not like fuel injection systems where there is a pressurized fuel return system. What the injectors don't need goes back. Not so with a carborator. The 1980 still had a caroborator, but some of the metering was computer conrolled.
SInce your dropping the tank, looking into replacing the lines as well. After 25 years and knowing you get a bit of rain over there, your going to get rust!
Had a number of TAs over the year (1975, 1978 with the outlaw Pontiac cam 400), 1979 and now a 1995.) Read about the 79 and 95 at my web site:
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Let us know how this turns out.