All slant six engines had solid lifters until the 1981's came out. Ma Mopar changed them to hydraulics as a nod to the exhaust emissions druids at the epa.
I guess the millions of slant sixes with mal-adjusted clearances on their solid lifter cams probably have a dirty exhaust signature on a meter...
Ma Mopar continued to make that engine with hydaulics until 1983 in cars, '87 in trucks and '87, as a marine engine.