Yes, the Opel Manta [B] looks a bit similar In fact GM cars often look rather similar. Thus it happened that I was mistaken here as well The Oldsmobile Starfire (1975-1980) is almost identical to the Buick Skyhawk (also 1975-1980), and I just found that this one-piece grille belongs to the Buick...
Opel Manta [B] was initially designated the Opel U-car Coupé replacing the U-1930 Manta [A] (U-car 2-door and 4-door was the Ascona).
GM North America then determined it would be a 'world-car' project and would also be built in North America, renaming the platform 'J-car'. Opel remained the design source for some time until gradually GM-North America carried out so many design changes there was little interchangeability of parts. So essentially they took the basic Opel design and re-engineered it for the USA.
In the US,it was based heavily off of the Vega.And just like Vegas,about all they were really good for was buying it used & cheap & building a Drag Racer out of it!!