Overflow
I'm with BillyZ. Try to find out if the entire tank is tempered. If only the bottom is, have the back of it drilled up at the top. Install bulkheads with 90 degree elbows on the inside to adjust the level, and plumb as a normal bottom drilled tank. I'm not a big fan of overflow siphon boxes myself. To many issues with flooding if the siphon stops for me. Besides that, if you have a glass company that can drill it, or you want to try it yourself, It will be cheaper. Even if breaks while being drilled, a new 75g is pretty cheap. Good overflow boxes are expensive, and just don't have the reliability to suit me. I've tried them on my 125g for awhile, but just could not get past always wondering when they might draining while the return pump kept filling. My tank is built into a wall in my basement and can't be removed for drilling so I finally chickened out after 2 months of worrying and went back to using 2 Emperor 400"s and 2 AQ 70 powerheads.
I now have 2 600gph overflow boxes @ $65.00 each, 1 1000gph pump @ $100.00, and gobs of misc. plumbing stuff boxed up and not being used.
That's how much I didn't trust the stuff.
Just my 2 cents though, Good Luck.