WE bought a 1994 Buick Regal with a 3.8 and 129k miles on it. After getting the car home 4 months ago we discovered the PO had put stop leak in the cooling system.
We did a combustion leak test and learned it was getting combustion gas into the coolant and so we are just getting the engine back together after replacing the head gaskets and plastic upper intake manifold. So the cylinder heads and lower intake were cleaned at a shop and we have a new radiator to install. All but 2 of the heater hoses are new. WE still have the old heater core and the 2 hoses that come off of the heater core in place. So basically the engine block, water pump, heater core, and 2 heater hoses are all that still has residue from the stop leak in it.
The residue is a brownish reddish color to it. When we first opened the system and the residue was wet, it had a sticky clay like texture to it. In 3 months we have had the engine apart, the residue in the hoses and radiator has dried into a gritty like dry clay texture. If you flex the old hoses and then tap them on something, small flakes of the residue come out.

I spoke with a radiator shop and he said to leave the old radiator on and flush the system really good a couple of times before installing the new radiator.
I was thinking if we run the engine with the old radiator (which is full of crud) it will contaminate all the new and clean parts.
I also understand that installing the new radiator now, that there is a good chance it will eventually get clogged up with the residue.
So I am leaning towards installing the new radiator and maybe flushing the cooling system several times in the next few months.
What do you all think?

"Here is a rough look inside the radiator and at the cap. we had wiped the cap with a rag and inside the neck of the radiator before these pics were taken.

I had similar residue in my system, but I attributed it to poor maintenance practices. The previous owner told me that they had never had a reason to put stop leak in it, but also never maintained it really either. I was dealing with corrosion and sludge.

I would flush out what you can by flushing everything with the hose until you get clear water out, so, block, radiator, heater core, all seperately. If you are still paranoid, put it back together with no thermostat, and perform a chemical flush. This will also give you a chance to leak check everything. Once you are satisfied with the chemical flush, dismantle and hose flush again.

Make sure you flush the new radiator good before putting it in too, after all the above.

Ideally it would be filled with a green coolant and distilled water mix in the end.

Diehards will tell you to flush solely with distilled water due to potential contaminants in tap water, but I ain't got time for all that.

Leave the old one in, flush to your hearts content, then put the new one in. Use 50/50 pre-dilute and done.

Thanks for the help!
I should have mentioned that I only have 2 sources of water here, well water and creek water. The well hose connection is before any filters and has a sulfur smell so not sure it would be good to use... I did buy 10 gallons of distilled water and used 5 back flushing water from the thermostat housing out through the water pump outlet but since we were just pouring it in, not a lot of gunk came out. I have to either use the well water or get the car to a different water supply.
Thanks
Anna