"...I want to make this thing as powerful as practical, without doing anything to the heads of bottom-end if possible..."
To make more power, you gotta pack more air/fuel mixture into it, burn it efficiently, then get rid of the exhaust, efficiently.
So, for MAX power you'll need a 1050 Dominator, a KRE Northwind (or Victor if you have room), the correct solid roller cam, big tube headers, at least 3 inch exhaust, with X-pipe, and very low restriction muffs.
Any less, and you'll have less power. But certain compromises will make it more streetable. So, it all depends on exactly the balance of power vs streetability you want.
I'd go with a hyd roller from SD Performance such as the "Old Faithful" or "Road Paver". But that may not be enuff for you.
I'd stay with the headers you have, and a 2 1/2" system. But, as said, if you make the other BIG changes, the bigger exhaust system will help, too.
These are just my opinions. My set-up would be "mild". But you may want "wild". Everybody needs to build what they want, not what somebody else thinks they should have. I've learned on these car forums, that my old fashion, conservative ideas are not that popular today.
