I didn't originally list my own experience on a Chevrolet board, but since our leader has listed preferences, I thought some might find my experiences interesting.
Shop van used year round in Missouri with 300 CID six.
Purchased used with 80k miles.
Drove about 3 months and decided I needed stock in Shell Oil if the original Holley 1 barrel was retained.
Changed 3 on tree to 4 on floor.
Changed Holley 1 barrel to Carter 4 barrel (Offy intake). Hwy mileage leaped from 12 MPG to 22 MPG. And it certainly helped the power !!!
Reluctantly junked the van at 440k miles because the Missouri winters (Modot thinks we have to have salt and cinders for the drivers who got their license at xxx-mart) rusted out the body the THIRD time. During the time I owned the van (80k to 440k) the cylinder head was never off the block. I don't think it was before that, but don't know.
The only other domestic straight sixes I have owned:
Ford 213 (burned an exhaust valve almost as often as a polititian told a lie).
235 stovebolt in a passenger car (hwy mileage 12.3, but this before I knew the difference of Rochester and Carter). With what I now know, I am positive I could improve that significantly. Totally reliable.
Several non-domestic, but will not go further with those (I was bitten by the "buy-American" bug) at age 30.
Jon
