Just a note to check your 440. The 440-1 has different water chambers than the 440-3. If you do a motor job and just get the 440-1 head gaskets, these will block the cooling jackets around the cylinders. Also I have run into other gaskets that don't fit properly. Never realized that MOPAR made 2 styles of the 440, until I got my MH.
From my research, the 440-3 was originally built for the military, for use on water pumps. This motor is a high torque low RPM engine. Therfore being built for mostly stationary use additional water jackets were cast into the block.
As to the header issue, mine has Doug Thorley's feeding into a 4" exhaust. Power and heat are no issue since install. Actually mine now runs on the cold side.
