Last question first: I think your thermostat is failed, open. After 20 miles, you should be above 80C. My car 560SEL runs that anyway. If the engine is not hot enough, it won't run right.
Some other things cause rough idle in older V-8's, like injector seals and leaking air hoses. I have not kept up with what you have replaced on the engine, past the timing chain and guides.
Did you notice a steady miss or just rough idle overall? Could be a bad plug wire end also. Might take some messing around, pulling wires to isolate?
Not much help, I know. Good luck.
1989 560SEL (172k)
1989 325IC (122K)
2004 Suzuki Volusia LE (3500 miles)
2005 Yamaha Road Star (20 K miles)