The heat shield was a big plate of aluminum or pressed asbestos
sandwiched between carburetor and intake manifold, used on some late '70s to '80s applications to try to control hot start/hot idle problems. The fuel line mod works better than the heat shield did. Cold driveability: Choke, exhaust manifold, heat riser valve, thermostatic air cleaner, and coolant-controlled EGR system are your prime suspects.
The choke thermostat, choke thermostat heater, choke modulator, choke pushrod and choke pull-off all have to be in good working condition, hooked up correctly and adjusted properly. The factory choke thermostat cannot be adjusted; the electric choke kits replace the original thermostat, modulator and rod with new fully-adjustable ones. Choke pull-off is adjustable and replaceable.
Not sure what you mean by making "a stove pipe connection to the breather". The breather is the can-looking device at the top rear of the valve cover, with a 5/8" hose conecting it to the air cleaner. The stovepipe runs from the sheetmetal stove surrounding the centre collector area of the exhaust pipe to the connection on the underside of the air cleaner's snorkel snout (maybe you were calling the air cleaner the "breather"?). Check that the air cleaner control hoses are routed correctly according to the factory service manual diagrams. Check that the hot/cold air damper door is in "hot air" position until the engine is well and truly hot, and that it's in "cold air" position after that. Check that the damper door actuator (round device atop the air cleaner snorkel snout) is in good condition and doesn't have a leaky vacuum diaphragm. Replacement air cleaner temperature sensors are available.
If the exhaust manifold is cracked or the intake-to-exhaust manifold gasket junction leaks, exhaust will be sucked into the intake tract when the air cleaner is in "hot air" mode. This will really do a job on driveability, so check carefully for cracks and blowing leaks.
The exhaust manifold heat control valve needs to operate correctly. Get a can of 4318039AB solvent from your nearest Mopar dealer and apply it to the valve. Liquid Wrench, etc. aren't substitutes!
The EGR system on this truck, if it is (still) so equipped, is a coolant-controlled system. There are temperature-sensitive control valves that prevent vacuum reaching the EGR valve until the engine coolant has warmed up. If one or more of these valves are defective, the EGR system will be operative with a cold engine, which will have the same deleterious effect on driveability as cracks or blowing leaks described above.
What is the condition of the carburetor on this truck?
