Anything is possible, but is it practical? For a given performance level, the V6's that are used in a G body will always cost more to build than a V8, and be less streetable. The problem is that these are old engine designs (some as old as 55 years!) that do not have the same level of efficiency as a newer engine, like a Nissan VQ37DE (370Z engine). Compared to a Buick 3.8 that has 110 hp, the Nissan 3.7 has 300+. Both are N/A, but the modern design has things like 4 valve heads, etc. To make a similar level of power from the Buick, you would have to sacrifice a lot of driveability, and possibly the ability to run on pump gas. So, if you want a V6 that has reasonable power, good fuel economy, runs on pump gas, can be found in U Pull It Yards ( The VQ came out in 1995) and is built in a RWD friendly orientation, get a Nissan. If you want a domestic V6 with similar features, and are willing to pay real money get the one from a new Camaro. And lastly, if you want a decently powerful V6 that bolts in to a G body chassis, find either a wrecked late turbo Regal or a GMC Syclone/Typhoon. However, the most cost effective option is a v8.
As for what can be achieved with simple bolt ons, I would say a Buick 3.8 could see 130-150hp. With internal work including more compression and ported heads, maybe 200-250hp depending on how much compression and the heads used. Or, just swap in a 96+ F body 3.8 for 200+ hp. It's a redesign of the engines in our cars that is far more efficient.
