Rpm is mainly depends on head flow for any given cid, if you threw on your 340 top end as is probably lose 500-800 rpms off the top. Porting the heads and maybe intake and larger cam will give back most/all of that lost rpm. A 408 can rev just needs the parts to support it.
If building a 408 forget about hp, where do you want it to make peak power 5500 ? 6000 ? 6500 ? Rpms
Then build to make her breath to that rpm (500 hp is around 6000 rpms depending on efficiency) you probably want enough port size to carry on 500 rpms or so past peak for shift points.