There is also the Girling IIA type which is fitted on HA SL with Discs. They are also on early escorts and Mk1 cortina, and Jags, but they are quite expensive, and a lot bulkier than the moggie ones.
You will need and adaptor off the inlet manifold for the vacuum pipe as well, and a larger plastic reservoir on the master cylinder top.
Some HA manifolds have a plug in them, just below the carb, so an adaptor can be screwed in
There is also the possibility that a larger bore master cylinder would be needed!! Worth checking before splashing out on a servo kit!! Seem to remember something about 17 and 19m/m bore sizes, or was it 19 and 21? but I may be wrong on that!!
If you do go down the Moggie route, they are a fair bit cheaper, but you may also need an in line one way vale, although some come with the white valve in the chamber. The Girling one has a screw in one as part of the servo chamber.
There is also some extra brake piping needed to be made up, depending on where you sit the remote servo.
Luckily my HA has discs and servo from new, but when I had a servo problem, I used Powertrack limited,
www.Powertrackbrakes.co.uk. , Leamington Spa I believe.
They were excellent.
The one way valve on the Girling servo failed on mine, and they are like hens teeth to find, and horrendous price when you do. So I considered converting to the other type.
However, I purchased a repair kit for the servo, and a screw in pipe for the servo, plus a one way valve in line to the manifold, and all is well. Total cost around £50, but that was about two years ago!
Good luck with it.