I have read some posts in other forums where people say they know someone running 500hp/1000lb ft w/upgraded clutch and the transmission holding up all day even though they are only rated for 520lb ft; conservatively.
I would be happy with 400/800. I know Valair clutches for the ZF6 get up there in ratings. Their high end single disc clutch for Ford 99-03 ZF6 is rated for 500/1000.
I will talk to multiple performance (Rosewood,Beans,Swamps) companies to see what they recommend.
I was just curious as to what people here have done when bumping up the power in their trucks with a ZF6.