/6 headers

There are no "off the shelf" headers available for the 170 motor, in a 66 or earlier "A" body. Any header for a slant six will fit the engine, the problem is interference, with chassis parts. The 170 engine is a "low deck" motor and the ports on the head sit 1 inch lower in the engine compartment, so the whole header sits lower. I can't say if the "power pipe" would fit or not. For the sake of this discussion, I would like to refer to the power pipe as a "Tubular Manifold".
There are slant six headers on the market that can be modifed to work with a 170 in a 66-down "A" body. I have a set of modified Clifford "shorty" headers on my 66 Cuda with a 170.
Mr Twister is correct, about stock manifolds, from a 225 fitting a 170. All stock slant six manifolds will fit all slant six heads, and will fit in all body styles.