Norman,
I should think that the jetting arrangement on an SU would make it far less than suitable for a propane conversion, and I'm sure there are units out there much better for the purpose. I won't say the SU is impossible, because I've seen people do remarkable and strange things to prove a point, but it's not the direction in which I'd head if I wanted a gaseous MGB.
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon