Congrats on the find! Gonna be a sweet tank for sure!
For the closed loop, keep in mind that most tanks have a tempered bottom. If you try to drill this, it will shatter. Find out if it is tempered before drilling, if you can't find out (might be hard on a used tank) don't take the chance. You should be able to have a similar effect from having the bottom two near the bottom of the tank. You could extend Loc Line to do this.
Also, the Dart will require a 2" bulkhead that needs a 3" hole. I believe the Snapper does as well. I'd follow this guideline for sure. That said, a single Dart on a 220 with 3' of head will give you 13x turn over. Consider adding power heads, upgrading the Dart to a Hammerhead or have two closed loops.
Oh yeah, subscribed