Foaming in Radiator

I just re-started a 70 340 that hadn't been started since 1987; it and the radiator still in the car, stored in an unheated shop(north Fla). Drained coolant, looked ok, refilled with anti-freeze. Fired it up and let it run several times (which included the 2000 rpm cam break-in, after I found the hyd lifters bleed down too much to my liking). Checked the radiator- brown crap; redrain. I was swapping trans, so I replaced freeze plugs in the bellhousing area-crud in the block. Then found some side freeze plugs that were suspect, with rust. Popped them out, and the crud I pulled out of the block was amazing. I think that wet crud sits against the freeze plugs, and rusts them.
Flush it good; I read that muratic acid works good. I've only seen foaming on low coolant- brown-dirty?.