my ramcharger has over 200,000 miles on it, and the valve covers have never been off....360 cubic inches of non-magnum mopar V8.
amandas truck has around 125,000 miles on it, and hasnt had any major problems, other than the water pump. however, i do know that people have had problems with the magnum 5.9.
the 318 in the mirada had around 150,000 miles on it, and i was racing it, not letting the tranny drop into 3rd gear, stretching 2nd the whole quarter mile...i am the second owner, and my dad was the only person to work on it with the first owner. in his records, he could only find proof of two oil changes between 1983 and 1994, when i bought it...there was so much engine carbon built up, the lifter valley was completely packed full, with narron tunnels through which the pushrods ran...and my old D150 (totaled) had the same 318, and it was around 150,000 without a problem when totaled.
in short, the older small block mopars. ive seen too many 3.8s with problems when i worked as a tech...and ive seen too many of the 2.7s fail.
ive not had others, at least not while ive been old enough to know what was going on...