OK, what's wrong with the auto trans? Is it faulty?

The diff ratio is the same for auto & manual so at highway cruise the RPM will be essentially the same since the torque converter will pretty much have locked up at those revs.

Auto's LOVE torque.
A15's mainly make torque, not horsepower.
The 1200's trans is a Jatco 3N71B, same as used in the more powerful L series powered models.

Here's a plan
1. Get crossmember from a 1974 or later Ute as these models used the same engine mounting positions as the A15 does. [different from early 1200 engine]
2. Install crossmember & engine. [it's almost a straight out bolt-in, ..... almost but not quite with alternator wiring & the engine mount bracket on the block on one side needing attention]

3. Fit rear wheels & tyres of increased diameter to try & increase the overall drive ratio. It's sort of like jacking up the diff ratio from 3.9 to 1 to perhaps 3.7 to 1
I recomend 205x60x13 on 6" rims as these fit nicely under my auto equiped 1200 coupe

Drive & enjoy. If still not satisfied then go with a twin SU type induction as these make good power & still have good vacuum. These Jatco trans' use a vacuum modulator to control the shift points so good vacuum is a must.

You will be amazed at just how much the driveability & performance will be improved while giving your left foot a good old rest.

Converting to a manual trans [you will need a 60 series] is a pain & in my view, will not yeild the results that you seek.