Since I know nothing about GM car parts interchangeability
... can you swap in a manual transmission and a diesel?
What motors and transmissions are on the 'easy' direct bolt-in swap list?
What is on the "it'll fit but it requires fabrication and modification" list?
Since the existing aerodynamics are decent and you can buy the chassis for cheap this could be a great test platform for building a super efficient highway cruiser. Good luck, this sounds like an interesting project.
Tim