the 3rd round of electrolysis did the trick...
I was able to get the parts from the known good doner 870 into the rusty 870 receiver.
In the process the ejector spring broke, but that was really to be expected.
The receiver is much more pitted than it appeared on initial inspection.... it has a rust type similar to the 1991A1 blood damaged evidence gun, not the buried early 1911. Deep pits surrounded by relatively unmolested surfaces.
Now its a matter of tracking down the following:
barrel (too pretty)
action tube nut
mag tube spring
shell stop springs (both sides)
trigger housing pins
topfolding stock (ugly)
ugly for-end wood
she is coming along very nicely