I'm not positive where the break point is, but early engines are odd fire, and newer engines are even fire. The even fire are much smoother. The point of the switch is the late '70's or early '80's.

Also, because you don't specify, I'm assuming you are talking strickly Buick, correct? Chevrolet had a 229 V-6 (also 3.8L), and I'd definitely stay with the Buick 231. Of course, if you can find one, the larger LaSabre and Electra models used a similar 4.1L (252) that can use many of the same parts as the 3.8L.

Another question would be the amount of horsepower you are looking for from your combination, and assuming your reasoning, fuel mileage? There a quite a few places that make performance parts for these engines. Let your fingers walk thru the internet search function and you'll find the parts you seek.

Just something to keep in mind. Newer cars come in various combinations. Some blocks are specifically set up for FWD. But, the internals from newer blocks should interchange. Crankshafts from intercooled turbocharged and supercharged engines I believe are forged.

If you look at Chevy stuff, stick with the 4.3L, as this is basically a cut down 350. Pistons, oil pumps, valve train items, etc. with interchange with SBC stuff. The only bad part about these engines are the lack of aftermarket support for items like intakes and heads.

Good luck.