Another thought!
have you tried to reduce the exhaust heat?
My rig came with metal panels attached to the radiator supports, along the top of the frame rail, back to near what would be considered the firewall to funnel air over the exhaust manifold/headers and exhaust pipe.
Another thing to look at is the exhaust system. Originally there was 2 inch pipes coming off the manifolds into the wye pipe then 3 inch to muffler and tail pipe. Restrictions can cause heat build up in the system that would radiate outward.