'69 Dart /6 Firewall & K-Member Questions

Are you asking wether or not there is a different firewall for slant six cars? If that is the question the answer is no. The only choice for firewall was AC or no AC.

The picture you posted looks like a slant k to me. If it isn't that is a very very tall left mount perch for a v engine. Another giveaway is the coil low on the passenger fender right above where a slant dizzy would be. That coil location is a long way from a small block dizzy, but not too far from a big block.