Except for the lack of aftermarket support, I've always wondered why nobody uses the 239 pre magnum and magnum engines in place of slant six builds even though it would take some mount to K-frame fabrication. I once had an 1989 pre magnum six in a very porky W150 long wheelbase 4X4 and it was way ahead on torque at any point from idle on compared to any slant I've owned. A set of cheap small block truck 318 headers with a tube cut off of each side worked very well, and MP offered a manifold at one time. The cam would have to be sent for a regrind, though, as all where rollers. If one got brave and creative, taking a combustion chamber off the ends of a set of aluminum small block heads would be doable... There would be plenty of room for turbos with a v-6 in place in an a body though.
