I remember asking here on Moparts in 2004 if Magnum manifolds would work on an A body and at that time the speculation was the Driver side would be a bear. True Hi Po 340 manifolds are expensive. If they rival the cost of the TTI small block A body headers I would go with the TTI units. Here are the TTI shorty headers.
https://www.ttiexhaust.com/TTi34025.htm