A ‘76 looks pretty much the same as a ‘75, but you have the newer B21 engine and the M45 gearbox, in most cases at least. The earlier B20 engine is sturdy and proven as well, parts are available if you check offers for Amazon or P1800. The metric parts are very few used in the engine, pistons are different. If the car has little or no rust, I would choose it, mechanical issues are rarer and cheaper to solve than rust, and even later 240 can rust, very much in fact.
Michael