Need Help building a 400 for my 68 plymouth valiant

Well I have the 452 heads on the 400. I was wondering if the 906s are better. I'm not sure if they do but can have them put in.
I'm hoping to get close to 450 to 500hp. I have a 8 1/4 limited slip with 3.23 gear in the back. Would that be strong enough or should I be looking for a something bigger?
I was going to get the 906s cause they the gone through and checked.
I have a friend with a rebuilt 727 that's stock. I was just going to put a shift kit in it.
I've converted the front to big bolt pattern manual disk. from a 76 dart.
Thanks for the information. I'm just trying to get educated on all of this from Mopar guys that know their stuff.