The OEM aluminum radiators with plastic end-tanks can be serviced. The end tanks can be removed, the core cleaned and then new end-tanks re-installed (the plastic gets brittle with time and heat). Scott Boelman, an early author here had this done on his 1994 6.5TD. There were several inches of sediment in the bottom of each tank.

Following a thorough flush, I recommend switching to Dex-Cool if your vehicle is pre-1996 (1996 and post got Dex). Dex is a lifetime coolant. The inside of a 16 year old cooling system still looked like new when I replaced the water pump in my 2001 Duramax.

Dr. Lee switched the green coolant to Dex in his 1984 and I switched my 6.5TD Power Project as well. Both had an all brass/copper/soldered radiator. The 6.5PP ran to +300K miles while we had it without a cooling system service - on the original water pump.