Hi Bill,
The Kadett B and the HB Viva were definitely related (cousins?) but I’m not sure that I would agree that the Kadett was the progenitor of the badge engineered Viva. They were both designed to the same GM spec, but in those days there was much rivalry between Vauxhall and Opel. They ostensibly shared a floor pan, but one was metric, the other Imperial (American) In my experience, there wasn’t a lot that would interchange between the two. This all changed with the next generation Kadett C (GM World car), Vauxhall introduced a limited range called the Chevette. They managed to make it uglier than the original but still used their own mechanicals, which differed from the Opel stuff. Still not a lot of interchangeability as I discovered when restoring an Opel C Coupé. The Grommet may have been Viva based, but there were a lot of Opel influences in the styling, which caused me to waste a lot of time too.
Dave