1958 heater components under the hood that mount to the firewall appear to be coated with some kind of a textured asphalt. Does anyone have a solution to clean it to make it look it's close to factory new as possible? Is there a rattle can textured asphalt spray that could be purchased?

 

It sounds like it may have been sprayed with bed liner.

 

If it is asphalt, you should be able to wash it off with a rag soaked in kerosine or mineral spirits.
Use rubber gloves.

If it's bedliner then something stronger like acetone or lacquer thinner may have to be used.

 

That's the spray asphaltum (sp) sound deadener GM used in those years. It chips off in big pieces often revealing brand new looking paint underneath. I have a couple brand new-old gallons of the stuff. Went on with something that looks like a undercoating two-hose gun. One of the assy line movies on YT shows a guy spraying it on. This is how that wide looking texture happened.

 

No over spray on this firewall