Aluminum heads will be a waste of your dollars and disappointing at that - UNLESS you are going to build an all lightweight forged bottom end so the engine can spin upwards of 8,000 + RPM's, and of course the roller cam will have to be so radical that it won't be streetable.
Do a search on our forums to learn more about camshaft selection, engine builds, and head selection. Much of this has been covered here and good info is to be had by reading all you can.
NEVER, EVER, apply any small block chevy crap to a Pontiac engine. Its apples and oranges and if you or your machinist builds the engine thinking it is a chevy, you will be wondering why you spent so much money for so little performance returns.
Best bang for your buck is the 400 CI block, 1974 or earlier, and a 461 complete rotating bottom end assembly for $1,550 at Butler Pontiac Butler Performance - Pontiac Engine & Rotating Assembly Combinations - Featuring Eagle Pontiac Kits
. You will find much material on this combo here on the site. Here is where you CAN use the aftermarket aluminum heads, but figure on about $2,400 for a set of PONTIAC heads, un-modifed out of the box. Then to really take advantage of the extra flow characteristics, you will want to select a roller cam set-up, probably another $1,000. Add in all the machine shop work, all your other match components and pieces and I'd say the build you are looking at will set you back near $7,500 - $8,000.
With this much HP & torque, don't expect the transmission or 10-bolt rear to hold up for too long, so budget for upgrades in both these areas as well as suspension improvements and front disc brakes to stop the car.
As suggested to other new comers to Pontiac's, the best thing to do is educate yourself about the engine and matching components that will work well together rather than hurt performance. Once you feel you have a good handle on this, then sit down and put together a build plan to include a budget or pricing. There are also a number of great books on building Pontiac engines and these give you the tried and true experience of what works.