I was looking at this at last years NEC show and what you are suggesting appears to be the norm from converters, cut the bracket, riv nut and Sikaflex 252. But even then I didn't see any that had fitted the rear bracket far enough back to align with the "support" markings on the Fiamma box. I haven't fitted mine yet as waiting for warmer weather for the Sikaflex but I'm going to use the next available factory bolt hole from the hinge to fit my rear Fiamma bracket. Just my opinion but I don't see it making any difference to how it holds the box on the roof when its closed up but it will mean there is more "twist" in the box when winding out the awning as there is a lot of leverage then, so as long as I support the weight of the awning while someone else winds it out I don't think it'll be a problem and will save drilling the van and fitting riv nuts.
IF it does prove a problem then i'll re do it as a cut and shut. The two pictures below are of converted vans at NEC.
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