Hi Deezee,
The scuttle will be fine to squeeze in both sides, if you warm it up a little bit, first fix one side into position then push the other side in to meet the edge of the chassis rail.
The moulding was made to allow for the slightly thicker grp side panels and also so that it can be moved along the chassis rails for position. There should be 3 layers of matting in the flange ends so plenty of strength there.
All the best....AndyH
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