My advice is get a 292 and forget about all the rest,except 302 GMC that are scarce.
Gm improved them by putting 7 main bearings on the newer sixes starting in 1963 they are smoother running engines and cranks are steal in them till 1966, and best of all they will fit all GM Vans and trucks 66 to 91, so you will not need to do a micky mouse Job trying to make them fit.
If you want go with an old one get a 302 GMC make sure its not a 270 cause they look the same.
Those 302 GMC engines will still have value in the future them others 235 261 270 will end up going to China and comming back as action figure toys or Toyota Echos.
