Can I use B or C body manifolds fore better flow on my sleeper GTS?

The red primered manifold is a 67-68 c body. Comparing the design of the two by eye leads me to think the b manifold most likely flows significantly more over the C body version.

I measured from the rearmost exhaust bolt to the firewall and came up with about 2.75". The b body manifold is about 2.5" from the rear bolt hole centerline to the back of the casting so it is going to be tight. I considered cutting the b manifold off and the flange to tip the outlet inward and forward which would help (I think).