slant 6 question

I heard that the blocks around 1974 are best if building a stout racing engine. The side web is thicker or such. The early engines had 3 staggered core plugs and later ones 5 in a row. Sometime around 1980, the head changed to "peanut" style and lost the spark plug tubes. Slightly later (perhaps), they changed to hydraulic lifters (changes to oil flow from block). I would avoid the peanut heads because you can't remove the lifters without removing the head (same deal in my 2002 3.8L V-6), and you can't get a performance hydraulic camshaft (if so inclined).

There was a version of the 904 transmission, termed 999 maybe late 1980's. I think it has advantages like a lockup converter and perhaps 4th overdrive gear, both giving better mileage. Would be interesting if it bolts up to a 1968+ Dart. I doubt they changed the crankshaft hole or bellhousing bolt holes. Google on AllPar, SlantSix.org, and others.

Sounds like the 1967 engine can't bolt to your 1976 transmission (crank hole too small). I think you can even have fit issues between an engine for an automatic and stick-shift of the same year, like the crank hole needs to be deeper to fit the stick's throw-out bearing. You might do best to swap the 1967 head onto your block, after at least a "ring and bearing job", though might be smarter to overbore for larger pistons while "in there" if compression is low since new rings doesn't fix "out of round".