Believe it or not, I side with the purists on the subject of transmissions. I have an A with the T5 and one with a '39 transmission fitted with a Zephyr gear set. Also had an A with a stock transmission and a Borg-Warner o'drive. All things being equal and getting the best bang for the buck, I'd do the next A with a Mitchell o'drive and sycro transmission.
Hopefully Auto Restoration has cleaned up their act when it comes to installation instructions. Based on my experiences, I considered their kit as high quality, ill-fitting components.