I don't think replacing the crank is going to reduce the flex which is a quality of the frame.

I recently picked up a red, white and blue Technium Pro from the curb and have been riding it almost daily since the rebuild.

http://backroom.hardsdisk.net/technium.html

I like the feel of the bike and don't mind the flex at all. On the road, it feels like a good quality steel bike to me.