That's disappointing to hear about your experience with Mitchell. I purchased one last year to put in a Rugby. Far different install to a Model A. I'm also down under in Australia. Had numerous emails to Sue about what I needed. Spoke to Sue and Steve on the phone also about my requirements. Sue even went up and beyond and chased up the correct speedo drive gears to put in the unit, so that my speedo remained accurate. Nothing but praise for them.
Like Grimy, my Rugby has a 4.89 rear end. High 30's mph screaming its head off. Much better now with the Mitchell, 26% reduction, whilst retaining the direct drive for the hills in my area. Made my own linkages that bolt to the top of the gearbox, with the lever in between the handbrake and gear. Blends in very well, and amazing how many people don't see it.
Hope that your situation turns around.