Thanks for the replies guy's.
Thanks for the info on galvanic corrosion...I'm not sure I need to worry about it though. What was the concern there?
Looking down the length of my frame I could see I still had a foam wire cover stuck in there, and it was so far down nothing I had on hand would be able to get it out. So this came home with me from the local hardware store:
It was a bit of a wrestle but eventually I got it out.
I also noticed the inside of the frame was filthy, not surprisingly after more than a decade of use:
So I rigged up a 'cleaning brush' using wet wipes tied to my new grabber:
That cleaned out most of the old dirt/dust...clean enough.
I made up my mock battery, but because of the wires and cables still in the frame (which I don't want to remove at this point) I couldn't slide it down to the bottom bracket to fully test the length.
True to my original estimates this box was 580x35x70mm.
So instead I cut a single length of cardboard the roughly the same height and slide it down one side avoiding the wires/cables:
It turns out from the bottom bracket to the outside of the first bolt of the rear stay is about 615mm. To factor in a buffer I would say the most anyone should make anything to slide inside the frame would be 610mm.
This is just as well, as a re-measuring of my intended 5P 12S 18650 pack turns out to actually need 605x35x70mm.
This is just a mock up 5P group as I don't yet have the cells to make the battery.
The local hardware store actually did end up having some 1/4" x 3/4" UNF bolts when I looked today, so I went with those and some washers to take up some of the extra 3/4" length. These are high tensile zinc plated bolts.
I put the bolt heads on the inside to reduce the risk of any wires getting snagged or ripped when I slide them in/out. Since I mount my controller inside the rear tail slot and pull most of the wires through this area when mounting, it's important for me to remove any sharp snags in this area.
Getting the 1/4" diameter bolts turned out to be a good move as they fit well and there doesn't seem to be any movement. I don't know if this would be the case with M6 bolts.
Obviously the bolts I used are a bit too long, and I plan to order some shorter stainless steel bolts soon, but I won't change them over till I pull everything apart again to install the frame battery so in the mean time I might get some dome caps for these bolts so I don't rip my legs open on them.
I've still got to finish putting everything back together, but so far mission accomplished and I now know that it should be possible to make this frame battery thing work.