Front seat or back seat? Bench or bucket? Sedan, coupe or convertible? Would help if your question was more specific. I thought (but I may be wrong) that within the GM family of "A" bodies, the same body style has the same shape seat. I don't know for sure about the Chevelle, but I used 1970 GTO covers to redo the interior on my 1970 Skylark coupe, and they fit perfectly. Hope that helps...
These are the '69 Chevelle seats that I am putting in my '71 Skylark convertible. The pattern isn't correct for Buick but it is close enough for my needs.
I am just being sarcastic if you did not figure that? Why in the hell would you want a Cheveel interior in a Buick? Just buy a Chevelle if that is what he wants.