This solution is only for those people that have had this achievement glitch on them and it not unlock when expected.
There was a solution on this site detailing these steps, but it has been deleted, thankfully I had read it after I experienced the glitch and was able to remember the steps.
I am re-posting a version so it can help other people in the future as it helped me.
I personally left this as the final achievement in case it went wrong, but that is not required as you can retain your existing cloud based save and continue where you left off so no progress is lost
I would suggest allowing yourself a couple of hours all in and doing it in 1 sitting1. You will need to start a new offline game and delete the existing console save.
First go offline: All Settings, Network Settings, Go Offline
Now delete your console based Save: the long way is to go to All Settings, System, Storage, Manage Storage, select FH3 and press , Manage Game, Saved Data, Delete All
The quick way is go to FH3 via Games & Apps
and repeat the delete steps2. Remain offline and Start FH3
You will now need to play through the early part of the game until you unlock the second Festival site (makes sense to chose Outback if you can as it's close to the Blizzard Mountain destination point).
Play some events in the new site area until you unlock the first Barn Find Rumour (this happened at the same time as the option to upgrade the second festival site to level 2 opened)
As soon as the rumour appears you can travel to Blizzard Mountain, (you can travel to Blizzard Mountain sooner, but there is no benefit if Barn Finds are not unlocked)
complete events and drive on roads on Blizzard Mountain until you get the Barn Find Rumour appear.
At this point you should not have any active championships open as they should still be locked, and you should have more than 40,000 credits in the bank
Locate and activate the Barn Find, watch the full cut-scene and listen the full vocal about restoring afterwards
Immediately travel to the Festival site and pay the 40,000 credits to restore the car instead of waiting
Get into and drive the car from the festival site, at this point no achievement will pop
Reconnect to Xbox Live by using the Guide Button, Settings, Reconnect
option, so you stay in the game, and it was now that the achievement then unlocked for me, giving me the full completion3. Exit FH3 and restore your cloud save
At this point I checked the achievement had registered correctly by refreshing TA, but that's not required, but once I was happy I went back to the dashboard.
Still online, I re-started FH3 and had the option to load my current console save (left)
or my previous cloud based save (right),
choose the cloud save and you progress will be restored.
It took between 10-15 minutes for it to sync my existing saves, I'm around level 470, fully completed everything and have over 400 cars in the garage, so expect similar.
Once it had finished the sync I checked and it was as I left it the day before starting this, but I now had the achievement, the game completed and a smile on my face.
As always hope this helps and add or check the comments for extras others add