[Dominik]

In the early days of Shamrock parts availability (manual transmission) was a challenge in SA. As was legroom in their "Mk1". Legroom didn't improve much later on, nor did their knowledge and skills. Once automatic, always automatic...

In 2002 they refused to even upgrade to knock-off wheels which were cheap from WAW/Wheelcraft. Anyway, I digress...

Not that there weren't a few manual cars.

With my 6'4" I didn't really fit in any Shamrock. Even with the seat padding reduced to Zero.

All else being equal, as in no parts sourcing problems or space, a manual is the way to go in this car. All you wrote about being told it's easier on the drivetrain and there is enough power is all BS. They just couldn't do it.

You can adapt a Toyota 21R (weak, but can be upgraded), Jaguar (scarce), Toploader...
GILO in Johannesburg has adapter bellhousings. Go with what they offer.

But first make sure you can add a clutch pedal. And a clutch slave cylinder between bellhousing and frame rails.