For posterity - the very large flathead bit that comes with the wheeler torque wrench fits good-n-tight. So my BFS consists of this bit in a 12" screwdriver extension, placed in a 1/4" socket and then in a 3/8" ratchet. .
I tested the fit of the butt in the now cleaned receiver and large bolt. It seems like I can pull it tight with little effort now that everything is clean.
After cleaning the butt, I noticed a hairline crack on its tenon. Here is a pic. It goes all the way to the base of the tenon.
Attachment 105095
Look's like I'll be fitting a new butt. Thanks for the reference - I have read and re-read Peter's article whilst considering my options. A bit sad at losing Doug.
On a more positive note, the finish was mostly gone on the back of the receiver after scrubbing rust off, so I cold blue'd it with oxpho. I didn't strip what was left of the old finish off. After a thorough degreasing, it blended in very well. I'll do the rest of the receiver similarly.
For comparison to above.
Attachment 105094