273 Numbers Matching car. Should I rebuild or swap engine?

Thanks 273! Would you go ahead and harden the valves (I think it is) so she ((my wife) can burn any gas in it (Ethenol) instead of having to find stations with 100% gas? That was my intention. In my '70 440-6 CUDA I have 4:10 gears and get 6 mpg! On my Harley I ride to eat... in the CUDA I ride to find yhese stupid 100% gas stations which are rare! It's a real pain in the rear and a constant search. I really don't want my wife having to search for 100% gas all the time. Thoughts?

Probably the best bet would be 318 heads out later model car. 318 after 85 have closed chambers for better CR. 318 and 273 are practically the same head.