The 231 even fire is a great option in a flatty, and a lot of what lets you install a 225 dauntless works just fine (mounts, input shaft, adapter plate, BOP bellhousing, etc.). Note that while earlier heads will bolt on the water jackets wont line up, so stay mid '77 to 78 and up for parts. Make sure you get one out of a RWD car. The FWD stuff was mounted sideways, has odd motor mounts and I think bell requirements. If possible find a standard shift RWD so you get the flywheel and not a flexplate. Pretty sure the 231 is externally balanced.
The '77-'78 heads were part of the early attempts at smog stuff and the 225 dauntless got more displacement but couldn't breathe (Small valves) so HP went from 160 in the dauntless to 105ish in the early 231's. If you find a '77/'78 block with heads, try and source some '79 and up heads with the larger valves.
I'm pretty sure that if you google buick v6 you'll find wikipedia has a list of models to look for... I have one out of a '78 olds omega
