In the duratec:
6-6.5 quarts is MORE THAN FINE.
Some people even run 7 qts (

) during events like auto-x, etc.
Overfilling, technically, would lead to whipping the oil in to a foamy concoction, and would render it.. hmm... useless?

Anyway... if you have "only" been getting 6qts, then you aren't near "overfilling" it, at least when compared to what they are afraid of. STOCK specs are 5.8qts, but stock specs and SAFE specs are two different levels in this engine (unfortunately)