hi guys and thanks for all of the replies.
i think i'll go belts and braces and fit 3 lambda bungs, 1 in the y-piece and 1 in each exhaust header and concentrate on the petrol side of the vehicle before i enter into the scary world of lpg.
and in reply to your question bobtail84, the airspeed velocity of an unladen swallow is roughly 11 meters per second, or 24 miles per hour although i cannot confirm if that is a european or african swallow.
scimjim