The early 60's engines you refer to were in fact all alloy & started life as a V8. A V6 was derived from it & both engine designs were sold off when Americans didn't buy a lot of them.
The Brit's got the V8 & have been using it with great success in Rover models ever since.

I thought that American Motors got the V6 but know nothing of it after that.

The V6 that was used in the first generation of V6 Commodores in the mid/later 80's was a somewhat unpleasant cast iron cast-off from the Buick plant with its oddfire crank & it's attendant vibration at lower speeds. From memory there were plenty of oil leak issues too that seem to have never been fully resolved.

They were eventually improved with an evenfire crank & they seem to 'go' OK, but are not really a 'first choice' when it comes to transplants, particularly in small, light cars with the engine planted firmly over the front axle.