Worth the change?!

My suggestion is to keep the engine as is with the exception of popping the heads off and mildly port them as well as a gasket match and a good clean up of the casting flash in the ports etc.
That would make for a good daily driver type car.

Stage 2 would be to do the head work and install the 2.02 valves in them, and the bigger cam (with matching springs) only if you plan on running long tube headers.
The higher you plan to rev the engine and the more flow you put through it, the more headers will help.

Sure, you will see a lot of gains with Stage 2 if you keep the stock manifolds, but they are a bit of a cork in the system if you start revving it to 6,500 r.p.m. as you say you want to unless you extrude hone them an that's a lot of $$$.
I would hazard to guess about 40 H.P. is lost.

Have fun, and let us know how it turned out!