Yup that card will work just fine in a PCIe 3.0 slot.
Thank you for the quick response.
Please pardon my skepticism, but do you know someone who has tried it or do you have a source? When googling "backwards compatibility" "PCIe 1.0 card 3.0 slot" I did not find one source. Most people either wanted to do the reverse of this, or jump from 2.x to 3.0, not all the way from 1.0.
I am relatively convinced you are right; but, before blowing ~$700 on hardware I would like to 100% eliminate the risk of it being unusable on arrival.