Definately not as strong as a TH400. The gear carriers/gears between the two are night and day.A stock gear carrier for a TH400 will do 1200hp all day long and probably a lot more. C6 gear carriers are aluminium and won't do anywhere near as much.Loo,k at 460ford.com or 429-460.com as they only deal with BBF (funnily enough)
The general concensus is not how strong they are but how hungry they are.
If you wanna rebuild with roller bearing and other friction reducing components then they use the same amount as a TH400 and are about as strong.
Difference being is that there are a lot more (and cheaper) hypo parts for other boxes