300TD W123 radiator descale question

[300TD1982]

I've had my W123 300TDT for 2 years. Recently I changed the coolant but there is still rust/gunk visible whenever I have a coolant hose removed. The radiator is an all metal one, and it weeps a bit around the seal at the top. I have a new Nissens radiator on the way. I plan to descale the cooling system with citric acid, but I'd like some advice with the order of operations.

If I descale it with the old radiator in place, I'm concerned that the citric acid will turn the weeping into a major leak. This isn't a big deal if I do it all in the driveway, but I'd rather drive around with the citric acid in the engine (I hear that helps the descaling).

If I replace the radiator and then descale the engine, I'm concerned that I'll plug up the new radiator with the crap I dislodge from the engine.

What would you do, and why?
Thanks for taking the time to help on this.