[BR3W CITY]

I'm assuming the 6.3 in the title was a typo (since you have LS3 everywhere else).
Anyway, if your going to use the LS3 heads thats the L92 stuff, which has pretty excellent flow. You'll have to use the L92 style manifold obviously, just keep in mind that things like throttle body fitment and a few other little things may also need to be changed or swapped out to make it all play nice (if your starting with a complete lq4 or something).

I'd say leave the ls3 manifold on the top and spend the money on a cam instead of the aftermarket manifold. If you put the matching ls3 heads-intake-cam I'd say you could see 385-400wheel hp with a nice tune (possibly being stingy depending on what supporting mods are done and how rad your tuner is). With the flow the heads have, if you do go with a bigger cam you can see 430-470. There is huge variation with that though, some guys have seen like 540 crank hp from a properly supported L92 style head package and a very well spec'd cam and tune.