I am looking at buying units in the downtown area. on the 200th block of 6th street.

Pick a 10 block neighborhood.  

If it is block construction the engineers will make you beef up the slab and add piles to support the new weight of the second floor.

The smart play would be to build a two story wood frame structure on top of piers or concrete block in most cases. 35 feet is a pretty typical height limit in most neighborhoods, so depending on elevation you SHOULD be able to build two stories on top of the elevated foundation.

have seen this play with eastern Block women . 

Are you doing butcher block, quartzite or masonite for counter tops?

If you want a city, Philly can be done if you know where the blocks break.

Before myself and a few other builder developers jumped in.. it was hard to sell anything in those blocks of town..

Ideally I want a combination of the Burnham block duplex style but cleaner, and more usonian looking.

Keep in mind, most offmarket/distressed deals are traded offmarket before they even hit the auction block.