I know you don't want to do an engine swap, but please bear with me.
The 200 and 229 Chevy V6 engines are very under supported with aftermarket stuff. That being said, there are some things that you can do.
Your engine is a "semi-even fire" engine that does not share much with the 4.3 from about 1985-on internally. You can run late 4.3 Vortec heads like the guys do with the small block chevy v8. You can buy an aftermarket cam but it must be for the semi even fire V6. You can use the 4.3 aluminum intake and valve covers, and headers as well.
Unfortunately, when you are done you will barely be making the power of a stock 3.8 Buick with your 3.8 Chevy. I wonder, what is the reason for not doing an engine swap?
If you want to stay V6, even Chevy V6, a good 87-up 4.3 can be had from a 1/2 ton pickup from the 88-98 body style and it will drop right in and look just like your 3.8. All the accessories will swap. So you can still maintain your stock appearance.
Of course, for V8 you are doing well, there, too. The small block chevy uses the same accessories and motor mounts as your V6 so it is a very easy swap for your car.
Why don't you want to swap? I'm just curious because I run weird engine combos, too, but it is just for the romance of it. It doesn't make sense financially.
Good luck!
