Which engine for rebuild ?

[TexasC10]

My 70 truck had a year 68 inline 6 mounted that started smoking really bad 2 months ago and lifters knocking like crazy.
So pulled it out and installed a used (low-mileage but have my doubts)
250 engine from a 1978 Nova.
Was told it only had 80.000 miles but not sure....
Anyway the "newer" engine has integrated intake & exhaust manifold and now
it leaks anti-freeze around the rear section when I turn the truck off.
A small stream can be seen on the left side of the engine block...so
I guess it's time for a decision what to do next.
My question is, which engine would be best for a complete rebuild,
the 250 engine from 1968 with seperate manifold and intake or the "newer" 1968 250 engine ?
I called around to a couple of different machine-shops and was told the integrated engine usually has cracked heads and not worthy trowing money at.
So asking you guys for advice or comments what to do ?