Most of these across the US have been tracked down. Yes, there is a Giant Muffler Man Historian Group.

https://www.flickr.com/photos/darrensnow/3236113184/

See also Debra Jane Seltzer's Roadside Architecture site for many more Giants (made by the International Fiberglass Co. a long time ago).

http://www.roadarch.com/giants/ifcowboys3.html

OK, yeah, I have strange interests . . .