I'm starting a mild buildup on my 1966 Chrysler Newport. Not exactly conventional but why not?
What i have currently on my otherwise stock engine is a K&N Filter assembly on top of a 650 CFM Speed Demon with an Edelbrock dual plane intake manifold. Headers and dual exhaust leading out to dynomax super turbo mufflers from www.ttiexhaust.com

Now for my question. I've been looking for new heads and a cam, and didn't want to go spending extraordinary amounts of money on the edelbrock aluminum heads, so i have been looking for some 906's. Although for my application i have been told the closed chamber 915 castings is a better design. But also, i believe, and am not sure, that the stock castings may be 516's, which is also a closed chamber design. So for performance heads, would it be best to port and polish the stock and have them rebuilt with high performance parts? or straight up start with new heads?...

If it were me, I'de pull the engine and just start with a 440, any year and rebuild that.  No matter what you do, you are going to have to spend some money in those heads since they are for leaded gas.  The price of rebuilding a set of old iron heads is about the same cost as a set of new Ederbrock heads that are brand new, with brand new parts.  If you are going to spend that kind of money, just find a 440 and put the EDDY heads on that.  Spend your money once, not twice.

66 Newport:
Don't be afraid to build the 383. There is a serious shortage of good 440 blocks that are affordable and not total junk. The 383 in stock form produced only 15 HP less than the stock 440 (I have seen many actual dyno numbers, this is not based upon the factory numbers, even though they were pretty accurate also). The 440 had an edge in regards to torque, but weigh's more than a low deck big block.
The 383 is inherently stronger than the 440, lighter and revs more freely. It is a blast to drive with the right combo of parts. The 383 was quite often called the "big block" that thinks it's a "small block". It's unfortunate that Mopar didn't develop it further, but the cubic inch wars of the late 60's stopped development of the 383.
A set of 516 heads is fine with a stage I or II port (good up to 525 HP), but that's all the further you can go with 516's (Thinner walls than 915's). Good 915's are too scarce to find (Edelbrocks would end up being cheaper in the long run).
If you want to get sneaky, you could also opt to build a stroker motor with the 383. I have done it and put together a 432 combo that produced over 435HP& 480Ft/Lbs torque with a cam that only had 219 degree's duration at .050" lift.

I current have a 383 motor with a Hughes "Old Time" Muscle Car cam with 224 degree's duration @ .050" lift cam, 9.6/1 Compression, Hughes Stage II ported 516 heads. It produces 425 HP @ 5,600 RPM & 435 ft/lbs of torque  (This is very streetable combo and a blast to drive)
Hope this helps

I thank you for your knowledge because i have a 383 that i rebuilt is being hard trying to find to find infor on it  

It also has the best rod/stroke ratio of any big block chrysler and probably all muscle era big blocks for that matter.