Not sure what gauge your battery cable is but unless you have a serious short the cables typically would only get marginally warm after starter has been run. If you noticed smoke after the engine has run it is most likely heat from the exhaust manifold transferring heat to the conduit. I would consider the newly painted metal conduit (even though it is high heat paint, some volatiles still have to 'burn off') or the asphalt sheath or combination of both.
PS. I do not know if the conduit was originally painted in 70, my 65 is looked to be cad plated, this helps to reflect heat vs dark paint absorbing heat.