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ok, so do I need to replace the heater core or radiator or will a bunch of good power flushes get everything out in time?

I do not think you have need to replace the core. but a good flush may well be needed. Granted it is a bit of a PIA to get to the hoses for the heater but get both of them off and prepare for a good spray under the hood.

You need to flush the core both directions, The back flush will dislodge a fair bit of crap. I flush them both directions a number of times.

I would expect the block has sediment in it too. Probably similar to what is in the reservoir.

Cheers, Charlie
If It ain't broke, Modify it!
87 Montero turbo Converted back in Spring1989
95 Montero SR 3.8 DOHC Only one?
93 Pajero 3 door 6G75 Mivec with paddle shifted 5 speed
Then a Gen2 SR with full coil independent suspension.