The biggest Buick negative is the oil pump design. That cast aluminum timing cover with steel gears is not exactly one of mankind's better ideas. Yet, Buick, Cadillac and AMC all used it and on every one of them is becomes an Achilles heel. Also, AMC engines were far better made than their Chevy counterparts with better head flow, high nickel blocks and tighter machining tolerances. They also have one of the worst oiling systems, especially compared to the SBC which is one of the best. Having killed a set of bearings and a pump in an AMC in around 15k miles has made me very leery of that oil pump design. Yes, I am a bit biased because I roll with the 55 year old Chevy design in my car, but I by no means feel that it is the pinnacle of automotive evolution like some of the old farts do. There are however, more solutions to common issues when going with a Chevy. If I were doing a swap, I would start with a newer block as some of the older ones are pretty crappy.
As for a Nissan in a G body, I would do it in a heartbeat with the R35 GTR's twin turbo V6! Heck, even the older R32,R33,or R34 Skyline straight six with twin turbos (RB26DETT) would have a place between the fenders of my Olds any time someone wants to donate the parts for the cause. It would be far more different than anything anyone has done in one of these cars an old pushrod V8 from the age of the dinosaurs!