I'm pretty sure that in the early-60's as the first "Econo-boxes" (X-body, which I think are great-cars) were coming into production, Buick and Olds shared the same "All aluminum" 215 V8. You could still order a 331 Old's small-block too, but there was apparently no Six-Banger for this car for 2 or 3 years, so this was the "Base" engine. Pretty much the same as the Rover-V8 too. I have a 62' 4-door F-85, and its got the 215, and the distributor is at the front of the motor, like a Buick. These engines were also used in Buicks too, the only difference is the heads on the engines, but they interchange, so they use the same block and rotating assembly, a very popular swap into MG's here and in England (With the 215's cousin, the Rover V8, which was the same Engine, just cast-off when the 215 crapped out in 64' or 65', when the X-Body was up-dated, and the F-85 became a full-size car again, and this motor was replaced with Six-Bangers. It was probably needed over there, where the Bigger cars needed more muscle, but at a lower price and more economey than some high-end sports car engines...)
But they did cross-breed in the late-70's too, like everyone said. They also did the same thing with Six-Bangers in the 60's, where the Pontiac and Chevy Sixes were the same-block, with different heads. Also in the early-70's I think some B.O.P.'s got Chevy Sixes instead of their own ones, probably just from production problems or something like that.