Don,
You look to be well on your way. I think the difference between drilled back and corner overflow is a bit of personal preference but drilled back provides more space for scaping and fish!
The only thing that is not clear is filtration. I'm assuming you are planning on a separate loop here and this is simply to automate water changes. However, if you are planning on purchasing a custom drilled tank anyways I would suggest that you incorporate the filtration system into overflow/return drilling. You can also put the auto water change design into a filter system and have about the same amount of plumbing to pass between your 1st and basement floors. Under this design I would add more holes, at least one more overflow and possibly a return.
One thing that I do see that is could be a problem is this, how will you prevent siphoning off the entire tank when the mag pump is not running? You will need to put the valve on that system on a normally closed solenoid to make sure that you do not drain your entire tank back down to your barrels. Check valves are another option but not fail proof and of course the larger the head the less reliable the check. If your design has the check in the basement I would worry. The other option is to put that return at the top of the tank with an elbow pointing down and drill a siphon break just below the bottom of the overflow hole. This is a fail safe method for making sure you do not drain the tank but could be a bit noisy at times if not setup correctly.
A forum that is very good for plumbing designs is Monsterfishkeepers.com, specifically CHOMPERS and Pharoah are very good at offering critique on plumbing systems. I would think hard about how you design the overall system and have some patience with the design. I have been working on a design for a 250G for several months now and am just getting to the point where I think every task will be automated and the tank will be power fail safe. These two criteria are very hard to fulfill simultaneously, so take some time and work it all out.