Where's my oil going??

[OKC190]

This is not a Mercedes question, but I need a bit of help on my sons engine. We recently rebuilt his 400 chevy small block. It's been running great, with one issue I can't seem to track down. After about 1000 miles, it is using about 1 qt in 400 miles. I assumed the rings hadn't seated, but they should be after 1000 miles. The engine runs great, doesn't smoke. I checked the plugs this weekend and they are all a very nice tan, no soot. I'll be doing a leakdown check this coming weekend, but I thought that someone might have a suggestion.