some are
yes
Interchangeable with what? Nearly all of the body and interior parts interchange with a 1971 Chevelle. Most of the body and interior interchanges with a 1970 Chevelle as well. Rear axle interchanges with ANY '68-'72 GM A body. Engine and transmission interchanges with any Chevy from the 1950's up to the 1990s at least. Front suspension also interchanges with ALL GM A bodies. All small block radiators are interchangeable. All big block radiators are interchangeable. It all really depends on what part you are trying to use/replace.
no
Some parts in GM & Ford vehicles are made by the same manufacture to GM or Ford Specifications. Things like trim pieces, steering wheel, tires, wheels, seats, some emissions parts, etc. But by and large parts are not interchangeable although the same company may make parts for both auto manufactures.
yes, all you need are the proper mounting brackets from gm. yes, all you need are the proper mounting brackets from gm.
Yes. Power steering fluids are interchangeable.
Seats from any other GM A-bodies (Buick Century, Olds Cutlass Ciera and Pontiac 6000) should fit.
If they're the same make and model, most likely.
Yes, they are very different.
I remember: Ford, GM, and Chrysler models
From 2000-2003 are the same engines in the GM family. Also look out of the same year of Pontiac and other Gm models will work with minor changes.