Hey, the angle is indeed 90 degrees! I measured it just a bit ago, and hopefully this will result in some serious quality building.
My suggestion: install dolly castors. The castors on my 612s, while functional, are too small, and make me nervous when transporting my gear. I'm currently debating if it's worth modifying it, or if I should keep it original. I don't intend on selling it, and I think anyone who might ever want to buy one(should I ever come to that decision) would probably wind up doing it themselves. Ohh, the questions that plague...