Dodgeman's got it right. The A15 is the way to go. The A12 with auto is functional but not performance minded at all.

I had an auto equiped sedan many years ago with the same setup that Dodgman is building now. Was an A15 with a mild cam and had a GX head with the SU carbs and GX exhaust manifold. This car was lots of fun but still slow off the line. Didn't get going untill you got over 20 Mph, then it went great. You could power break the car and get the right rear wheel to spin. Kind of silly but fun.

In that time frame the 16V Volkswagon GTI's had just come out. From 30 Mph up to maybe 80 Mph or so the Dato could keep up with the 16V GTI's and pull away from the 8V GTI's. Opened quite a few eyes...

I had looked into the 4N71B trany also. Looked like it would work with the exception of having to have a short tailshaft made. In the larger cars the 3 speed autos came with the long tailshaft housing. When the 4 speed came out the over drive unit was the same length as the difference between the short and long tailshaft housings. The 4 speeds came with the short tailshaft housing so they were the same length as the 3 speeds. In the 1200's the auto already has the short tailshaft houlsing so the 4 speed is longer...

Have fun!

Gary