In your immediate future, I can see you spending $1100 to $1500 for a really nice set of heads and getting absolutely no benefit what-so-ever out of them......because you don't have any supporting mods that will allow you to take advantage of a heads upgrade.
NOTE: The heads are not even close to being the 'weak link' in the LT1. As a matter of fact, in a stock LT1, the heads are about the best thing going.
You'll want to do a LOT more research before you spend your hard earned money on upgrading your car.
To elaborate a little from what KW was getting at, catback and cold air intake are only the tip of the iceburg of what need to be done to support any head/cam upgrades. You need long tube headers, stall if it is an auto, clutch if it is a stick, gears (3.73 if auto, 4.10 if M6), valvesprings, NSA rockers, pushrods, guideplates, ARP rocker studs, rear suspension, etc. And on top of all of that, A TUNE. Not a hypertech or diablo programmer POS waste of money, a REAL TUNE, a custom tune, whether it be mail order or dyno tune. All the supporting mods listed above will run you EASILY over $1500, if you piece it together with used parts, you can save some $$.
As far as cams, the 306 is too big for a stock head/stock bottom end LT1. Period. End of discussion. Unless you are only putting it in for sound, and dont care about daily driveability, then fine. The stock rod bolts will also let go if you are spinning that cam to where it wants to be spun, 6700rpm. The 503 is a good stock head/stock bottom end off the shelf grind.
Dont do the heads unless you are doing a cam first, or at the same time. Heads alone will net almost nothing because they are not the weak link in that pair, the stock cam is. You can either spend $1200 in heads and gain 5-10hp, or $1000 on a valvetrain and cam and get 40-60hp. My favorite grind (right now) for a stock bottom end LT1 with stock heads, is the AI 226/232 on a 110. One guy made over 370rwhp with just that cam and thinner impala head gaskets.
USMC, your compression ratio is too low if you still have time to change it, head for something closer to 12.5-12.8:1. Just make sure you have someone who knows what they are doing tune it.