Is the car misbehaving? If you're concerned there's gunk in the system, there are few steps you can take. First fit an inline fuel filter (because whatever rubbish you're about to dislodge you don't want to make its way into the carbs and the engine). Second, use a premium fuel as these often have cleaners in. Third, you could go down the additive/cleaner route. Fourth, you could take the tank off and try and clean it yourself. Finally, you could send the tank away to a company that will clean it for you. Whatever you do, chances are you're going to upset an awful lot of sediment. Personally, I'd start by fitting a inline fuel filter and not letting the tank go below 1/4. If it's misbehaving, then check the operation of the carbs, make sure that the air filter isn't blocked, clean and gap the plug and inspect the rotar arm and distributor cap. You could also make sure the throttle cable can move smoothly.