If you were really intent on doing this sort of thing to your MGA, you can buy the parts and find full instructions on Doug Jackson's British Automotive web site.

I do not personally recommend the procedure for a daily driver car, as it requires irreversible cutting and welding modification to the original MGA frame. Suspension motion configuration remains essentially unchanged, and unsprung weight is not much different. For no holds barred competition there may be some advantage to using adjustable tube shocks, but this may also put your car into a different racing class. Read the rule book first. Many vintage racers get along well by changing shock oil viscosity of valving in the original Armstrong lever shocks to affect dampening changes.

If anyone who does not have a vested interest in selling these parts should happen to do this conversion, I would dearly love to have a user report.