Okay, I guess it is too wordy. I always think of alternative plans as I go along..
I got leery of using the old steel oil-fired boiler.. It's super old and could start leaking,
and we would be without
any source of hotwater.
So, the safe route is to use a new tank and install/use the A7 the way it was intended.
It should look somewhat like this, when I'm done..
Here's a pic of someone's install before inserting the HX loop.