The roof damage is a little scary, but how is the car otherwise? Is the underside rotted out? Does it have any real rust on it in the floors or elsewhere? If it is a salvageable car, I would probably do the paint and interior first, then worry about the drivetrain. That way, if you get cold feet, the most important parts are done and it will be easy to sell on for a decent enough sum. I also tend to see the SBO as an easier swap than the SBC ( and I have an SBC in my 85 Cutlass) as it lets you retain most of the original brackets, mounts, wiring, plumbing, etc. I don't have a problem with using a Chevy in an Olds, but I do realize just how much work it is to do it properly.