340 or 408?

Do you plan on running the quarter mile ?

If not proper gearing become more what you like, you probably want the 408 turn the rpms your now 340 can at least with 4.56.

My guess is your probably around 450 hp, with porting heads and intake possibly more cam (never gave full specs) you should reach your goals but the powerband (peak tq & hp) will moved up around 500 rpms and depending on cam low speed might have to suffer more, how streetable is it now ? If you find it on the edge of streetability you might not like moving the powerband higher if you find it fine then might not be a problem for you.

As for 408, take the rpm powerband you have now and add power across the entire range is what a 408 is basically gonna do if you get enough air flow.