Asking for more input. Again.

Major rust or bend issues in the frame rails where the rear springs mount, center member where the torsion bars mount, and the upper arm mounts are more difficult to repair. Anything else can be repaired or replaced much easier.
The first place I look at are where the inner fender is sandwiched against the shock support. If those humps are rusted through I can expect front splash shields and the trunk extensions to be rusted before I look. These may be just replacable sheetmetal pieces but its cancer none the less and will need repair eventually. Bottoms of the doors get a glance. Look for yesterdays broken glass clogging the drain openings. That doesn't definately say the door has been wrecked, just a prompt to look closer for bondo in the doors and fenders.If the bucket seat tracks are rusted you can guess the floor pans are too.
I suppose I said "A solid foundation is the key" in 1000 words or less. LOL
As far as the engines... I wouldn't haul the 273 home for it. If it was a complete Commando engine I would want all those goodies off of it. A 318 that was built for unleaded fuel is a better choise in my opinion.
PS I want a set of those rocker mouldings !