Buick 350 gets my vote. Only tough part us finding one. Easiest install you could possibly do. I can give you a complete parts list for the swap if needed.
Pros to swap to a Buick 350:
The 3.8 is literally a 350 with 2 cylinders missing and a shorter stroke.
Everything attatched to the 3.8 will bolt directly to the 350.
350 Buick fully dressed is 75-100lbs lighter than a sbc.
Non adjustable valvetrain.
Easily accessible oil pump and filter.
Starter, timing cover, distributor (physically will fit), and water and fuel pumps are the same.
Frame and motor mounts are the same.
Bellhosing pattern is the same.
Exhaust manifolds flow well.
Piston are the same. You can use (8) 3.0/181 flat top v6 piston for easy 9.5:1 under $250.
Cons:
Parts cost about 1.5x as much but if you shop around you can get great deals.
Special attention NEEDS to be paid to oil pump setup and bearing clearances. No more than .002 on the rods and mains.
That's pretty much it. If you go Chevy it will be cheaper yes but everything I posted and more will need replaced or modified.
Plus it's damn cool to pop the hood and show off a Buick v8.