50 gallons preboil was derived from four separate brewing systems, most with 52qt cooler mash tuns. All the runnings were then heated to about 210F and then 35 gallons were dumped into the barrel and immediately lost about 20F. 20 gallons were boiled completely separately.
We didn't really know what to expect going into it so we first started with 1 full block (15 pounds). It hissed and bubbled plenty, but any boiling was very localized. We stepped it up to about 2 full stones at a time and we were getting a rise of about 2F for each cycle. We were worried about using up the blocks too quickly as we only had about 8 of them in the fire, but it turns out that they heat back up in a matter of minutes.
It took about 25 minutes to go from 190 to boil. Eventually we figured out that filling that deep stainless basket, plus one more stone was money. We boiled over bad once, then almost a few more times. I put most of the bittering hops in the side boil that we knew would roll hard for 60 minutes so probably about 1/3rd of the hops went into the barrel.
After chilling, we added the side boil wort into the barrel and it came within 3" of the top after leaving the last 3 stones in the bottom.
The only problem now is that we couldn't chill lower than 65F and it's already fermenting. This unseasonably warm fall is not working for us. Certainly not an ideal lager ferment temp. I'm having the host open the garage door tonight and run a box fan to pull 35F air in.