You have plenty of rock; your sand bed will do a lot of the filtration. As far as the standpipes go, they work alot better if they're a larger diameter than the bulkheads; like if you have standard 1" bulkheads you should make 1 1/4" or 1 1/2" standpipes. I don't think you'll be able to fit 2" dursos in overflow boxes that are typical size. I'd go 1 1/4". Nobody seems to know why they work so much better if they're a little bigger than the bulkhead, but it's definitely the case. Probably something to do with friction in the pipe slowing the drain a bit.

Nice tank-makes me miss my 110.