Thanks, Jeff. Looks like I should be good for a pinch more than that.
After 5 hours in HD, I finally found that a 1.25" slip to 1" threaded reducer fits the drain opening very well. The big end, which will be the side in the tank, even has a bit of a funnel slope to it which will improve the
flow rate. I'm abandoning the overflow box idea and will be using an inverted L shape instead. The top of the L will be slotted pvc. With roughly 18" of 1.25" head pressure, the choke down to 1" through the tank bottom should flow nicely. The exit from the choke is also 1.25" into a huge 1.25"
ball valve, so hopefully I won't get too much cavitation and noise from that. I did a dry fit when I got home and it all looks real good and sturdy. Won't take nearly as much goop to seal up as the previous arrangement had.
The same fitting also just barely doesn't fit through the return opening. I'm thinking of just shaving the edges off the threads to make that work. (We're talking maybe a mm and it would clear.) If it does, I'll neck it down to 1" or .75" inside the tank so that the return pump won't have so much weight to work against. I'll just be doing a drilled t-shaped spraybar up top for that. Unfortunately, the bulkhead holes are too close to the rear wall to be able to use those handy disconnect fittings.
Had too much else to do today to get any of this installed, but am looking forward to getting it together this weekend and testing the drain rate. I'll be real happy if I can get 700gph out of it. If it does end up being 600 or less, I may drop back and order those drill bits. Really hoping to avoid having to run any powerheads in the tank. (even as nice as the Tunze are.)
Y'all have a great Christmas!
Cheers!
Dan