Matt-

Carlos definitely has 1" bulkheads inside the overflow boxes.
1-1" in each corner overflow. I have seen his tank which has alot of rock but it didn't appear that there was any type of dam or anything around the returns (I may be wrong here as I couldn't see under the rock). The returns are definitely outside of the overflow boxes in the corners.

As for dimensions of the overflow boxes, here is what Carlos said in a different thread:

The overflows are 23" x 5" x 5" each side has 8 opening, each opening is 1/4" apart and they 1" deep cut.
HTH.
Chris

Carlos- I will give you a call later today once my company leaves.