Hell, I bought my whole running, driving, slant six auto 1967 Plymouth Valiant Coupe with the original black Cali plates for $300. I sold the good 904 auto for $150 when I converted over to a four speed.
$450 for a slant six engine is a little high. I'd probably pay as much as $200 for a good running 1968-1975 engine with good even compression across the cylinders if I needed one, but likely I could score one for about $100.
My buddy and I have both bought good running 70's 440s for around $400 around here, him a 1977 cast crank and myself a forged crank 1973 engine.