Tim, why not pop to the factory and watch what they do there trimming wise?
That could be an option. The big difference of course with ally is that you can trim it in place using shears. With fibreglass I have to take the wing outside to cut it because of the dust.
There aren't actually that many absolutely critical dimensions as far as I can see, it's just the bit that goes against the bulkhead that needs to be tight. The rest of it is out of sight either under the bonnets, cowl, or body tub and can afford to be a mm or two either way.
Tim H. 1986 4/4 VVTi Sport, 2002 LR Defender, 2022 Mini Cooper SE
