dusty - 1956 Ambassadors came with Packard V8s and part way through the model year, an Ambassador Special with the new AMC V8 at 250 c.i. was introduced. This is the engine that is related to the Kaiser V8 proposal. The next year, they put this 250 V8 in the compact Rambler chassis to create the Rebel. This engine was intended for a full size car and came with a Holley 4160 4bbl so it was quite the opposite of what the typical AMC customer expected in 1957-58. The 250 was joined by a 327 in 1958 and sometime after that the 250 was replaced by a 287. When a new line of V8s was introduced for 1966, they came in 290 and 343 sizes and a 390 was added for 1968. By 1970, the sizes got increased to 304, 360, and 401.
The Packard V8 came with Ultramatic (S-P needed to sell transmissions as well as engines.) and the AMC V8 used Borg-Warner automatics.
GM had more than just the Chevy 307. Buick stopped making V8s in 1979 and a turboV6 wasn't going to satisfy traditional Electra buyers so, Buick started using Olds 307 V8s! Buick had been using Olds, and yes, Chevy V8s and even Pontiac for several years prior to 1979 so the announcement wasn't as a dramatic change to the line up as one might think.