Sorry, the access plug is on the head. I think it is in the same spot on most or all of MB heads. At the very rear of the head, on the intake side. Now of course this is for inline engines.
I'm not sure about V-8's. But I would look anyway. When you fill from a low spot like the expansion take, coolant will not fill certain passages. The air cannot escape. It's kind of like priming the pump.
'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)