I'd pick up that 360 ,and the trans ,if possible. 360 and Auto makes a great street/strip performer in an A-Body,when built right. The auto will be a much simpler installation as compared to finding all the 4-speed bits and pieces, but that's personal decision for you.
That 340 block is worth good bucks to someone who needs it for a restoration project. Is it just the block, whole engine, or somewhere in-between?
