Yeah, the plastic neck is one of the Volvo things, too -- mine broke in the driveway while I was messing about with the injection pump, fortunately. My sister's let go on the interstate -- mine was $350 after screwing around for a week trying to get a tank on it -- the rad was shot, too, after the new tank sealed. Sis's was $750 -- had to get a rebuilt head, hers was rather badly warped.
I suspect that is what happened to my 87 300D, too -- brand new rad and what appears to be a cracked head...!
I recommend anyone with an orignal plastic rad to try shaking the upper hose connection hard periodically, in the hopes it will break while cold rather than on the highway -- no way to shut the engine off fast enough to save the head!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!