How much power do engine mods yield?

I pulled this posting from the forum and was wondering how much extra power you're going to get with mods like this...


See if he will recurve the distributor for you too. That is a low cost high gain improvement.
I would do this in this order:
Cam and head work (dutra cam/shaved head)
Distributor recurve

2 bbl Supersix with the next $200-$300 you can invest.
Better throttle response and better pull up through the rpm's

Dual exhaust with the next $500-$600
Better mid to high rpm power

Is it a noticeable difference or just a slight bump up. My car is going to be a '65 Valiant 2 door and I'm wondering how much of a difference these mods would make along with adding a manual trans and better rear end gears...

I don't want to swap the engine, I really dig the slant 6!