That's a good question to ponder.
The 250 came out in 1966 and was phased out in the mid to late 80's. As Martin mentioned, the 250 with integral head had issues (cracking etc.). This head design came out about 1975 so the good Chevrolet 250 years are 1966 to 1974. But wait, if you are not happy with engines fitted with equipment to meet EPA, then the years can be shortened to 1966 to 1971. That's 6 years (inclusive) to chose from.
The 250 was one of the 3rd generation I6 engines so it received improvements such as modern hydraulic lifters, newer design rocker arms, spin-on oil filter, and etc. From the aspect of improvements over pervious generations, the 1966 to 1971 250 engines are "GOOD".
The 250 fits well in a narrow engine bay. So for previous generations of cars and trucks that came out with I6 engines, the 250 is "GOOD".
The 250 is designed for torque for hauling, farm work and construction etc. so it is a "GOOD" engine for a working truck.
So why did GM stop making the 250 if everything is so good? The design simply can't be retrofitted/upgraded cost effectively to meet federal emissions and MPG mandates.
That's not good and it's bit sad for us I6 lovers.
Comic relief:
But all is not lost. Jerry (Hot Rod Lincoln) is working up an Atlas I6 conversion kit for AD trucks.
Unfortunately GM pulled the production plug on this I6 generation (2003 to 2009) as well. That is "BAD".
