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

Just rebuild it to stock specs and add a small cam and 4bbl with small 500 cfm carb. With 3.23 or 3.55 gears.

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?