magnum manifolds in a early a

The pictures showing space between these manifolds and firewall would be deceiving in that they cant show the heat transfer. Comparing where these manifolds dump to the stock manifolds will at least give you insight to why the originals were made different in the first place.
The first thing I would try for a cure is a simple sheet metal heat shield which seemed to work in other places from starters to cat' converters, and many places under modern cars. If it reaches high enough to catch some air flow and 2 ears at the flange bolts stand it off far enough to prevent a wind noise at road speed, it could actually cool the down pipes too.
Nothing more than a gear screw clamp would secure the lower end regardless how long it is.