The 350c[lockup], in stock form, may do just fine behind 400hp.
But that depends on you and how rough you are with it.
The weak area with any 350 is the intermediate sprag race, which can break and leave you with no 2nd gear.
Usually when a 350 is used behind 400hp a hardened sprag race is a good choice as well as doubling the bushings in the sun gear, a wider bushing in the direct drum, a stiffer pressure regulator spring in the valve body, for higher pressure, a little calibration work to the vb
Again, if your not to tough on it, the 350c will do just fine.
But with gas at around $3.00 a gallon I think I would get that lockup feature working.