The picture of your mounted duals are Firestone RH 5° rims, they join in the center with very little metal to metal contact, this is the "Window Maker" rim. The picture of your front rim suggests that it as a continuous side ring with a similar small metal to metal contact. It is barely any better in safety. If these were standard flat base rims with a good split side ring, then you would merely have a dangerous rim.

You might try Hemmings besides the normal Chevy forums. as you said, the rims are out there but hard to uncover. Local on line auction has 6 rims with tires in the 10 bolt Chevy pattern that appear to be the continuous side ring version.