318 Duster Rebuild - pros and cons of keeping matching numbers

What's wrong with the engine ?
I'm just tearing down my grandsons 273, original std 67 bearings, rope main seal, no oil consumption, other than black yukky rocker arms, I shoulda left it alone.
Just wondering if leaving it alone is viable ?
"New" engine suggestion is concerning .
My 69 I bought new, wouldn't be #s matching without 3 sleeves over the years