we are going to add a fiberglass hood scoop to a steel car hood We have seen this done so many times, but also see them crack at where they mate later on down the road what is the best way to do this and what resin type should we use to prevent this from occurring

They crack because of the difference in the rates of thermal expansion in the dissimilar materials. I would think your best bet would be with one of the panel bonding adhesives like Lord Fusor or something like that.