Yeah, I've thought about an Olds motor. Pretty much any GM family will drop in with a careful selection of motor mounts and accessories: buick, olds, chevy, pontiac, caddy. All of them (except caddy) were used in the G-bodies. Of all of these, the Olds and Pontiac are my two least favorite. The Pontiac is pretty heavy and both the Poncho and Olds have small bores so they hurt for breathing. You are right, the 350 olds and the 403 have great dimensions, but really crappy bottom ends. The Caddy is one of my favorites: huge displacement, high nickle content, cheap cores, and not much heavier than many small blocks.
For every project I come up with objectives. My 66 Bonneville was going to be a tow vehicle, so I wanted major stroke, huge displacement, low rpms, and respectable mpgs. I used the caddy 500 with a mild cam and it makes about 600 lb-ft at 1500 rpms, and 390hp all before it hits 3500 rpms. I also used an 8-lug, 14 bolt rear axle from a 2-ton truck.
It hauls alot and gets 18mpgs.
This Cutlass is a daily driver, 100 mile commutes, 28k a year, and a few cross country jaunts each year. It needs to get about 15-20 mpgs, have very common off the shelf parts, and be easy to repair while having easy bolt on performance parts. So I'm looking at the most "common" possible engine. This screams for a 350 chevy, but I refuse to put a chevy in my car. That is too blah. The next most common would have to be a 302, so I'll either mod the 307 thats in it, or use a 5.0 ford.