I don't think they look horrible without a bumper but will need some adaptation.
Front has two beams coming out the front that the bumper mounts to. These come through where the grill used to be. If you cut and cap them, an original grill bolts right in.
The holes for the signal lights were cut bigger in the RB cars. The little "plinth" that the original signals attached to is gone. Some people push the old signal in from behind and it sort of fits. I had my body guy fill the holes in completely and drill for beehives.
Also under the bumper is a larger hole for air flow to the engine. I had it filled also.
Back - I can't recall how the bumpers are attached, but, the real problem is that they cut off the curved area under the tailights to make the rubber bumper fit. My car had been lightly rear ended so I had my body guy replace the RB valance with a chrome bumper valance, then everything worked for chrome bumpers.
not an easy conversion.