late model slant six head
$300 for a used slant is humorous. They would surely giggle once they had your money.
Later engines (~1980?) had a different size bolt hole at the rear rocker shaft. There was a special undercut bolt in that hole. Some thought it was for oil supply, but others said it was so the rocker shaft couldn't be installed wrong. I don't know if the tapped hole in the head varied. Anyway, read up. I don't know if anyone has installed the later hydraulic valve train in an early engine, if you desire that. I recall talk of having to drill an oil hole in the block or head. Probably less risk to stay w/ solid lifters & rockers.
Perhaps the biggest downside of the later heads is that you can't remove the lifters without taking the head off. Main upside is no more spark plug tubes to leak oil, though those are a tie to the 426 Hemi.