or use a 'jap' style boost controller with it's own valve, or escort cosworth 'amal' valve. plenty of posts on them in the past few months.

the electronic boost controllers do pretty much the same as what gareth is suggesting using simple pnuematic valves and tend to give a harder boost hit due to reducing wastegate creep, but have the advantage that you can adjust the setting more easily and in the car. they can also give variable boost setting dependant on gear selection, but you need a speed sensor for the controller to work out which gear you're in from the revs/speed. your shonky taxi doesn't have one, except for using the abs, which i believe is beyond your diy tuner capabilities