If you get GT40p heads, then you have to change your headers too. Plus there are significantly better options out there.
What is your budget?
IMHO, the thumper heads are not a good choice. Why spend $600 (or $700 with real springs) for a set of heads that, at best, are mediocre performers? They have small valves, mediocre flow, and not much room for growth. For a few hundred more you can buy a set of aluminum Edelbrocks, TFS, or other heads that already come with better springs, larger valves, rocker studs (not pedestals), better flow, and more room to grow - not to mention there is a significant weight break and better cooling capacity with aluminum heads.
As far as cams go - for a mild combination there are many off-the-shelf choices that would work. e.g. TFS1, Comp XE line of cams, etc. Make sure you upgrade the intake at the same time. If you want to save some money, buy a used intake manifold. They can be had all over the net, they don't wear out, and will work just fine for you. Look at intakes like Edelbrock Performer, TFS Street, Holley Systemax, etc...
Lastly, if you want to go faster for less money without all the work. Just buy a blower. The install is easier, they can be had used on the corral relatively inexpensively, (just make sure they have been treated well and all the parts are there) and they can be installed in a weekend.
Paul.