Is my car worth keeping the original engine?

Hello all,
My '67 Belvedere II has the original 318 engine in it, and has had maintenance items done on it over the years, but has never had a long or short block done. It has a bit over 42K original miles on it, and starts and runs strong, but shows it's age...a bit of valve train noise, shows a hint of light blue smoke at time, and never idles as smooth as I'd like it to. I haven't had to add oil between changes, and has no leaks. The timing chain, carb, transmission gasket, head gasket, and rear main seal are new.
I can just drive the car as is, but I've considered swapping the original block for a well-done long block, or having a long block done to the original engine at a shop.

I realize my car, as nice as it is, isn't a huge collectable. Would I be making a mistake by taking the original engine from it?

Thanks!

Author: BLUE DOBB