The sole reason the 5/16ths and 1/2 was even offered was because the 5/16th fits under stock 350Z strut bar. We don't have that issue, so it'd be wise to go with 1/2.
Tony from Motordyne wanted to market something for us to utilize even MORE volume. The test platform he made, he bolted to his 350Z and he loved the results and was talking and hyping this thing up like none other... He couldn't keep the hood of his 350Z on while testing this, and then when it came time to try it for the FX he found out that the difference in engine mounting and firewall placement made it a no-go for the FX. The firewall wouldn't allow it.
So go with the 1/2 for an FX
Tony from Motordyne wanted to market something for us to utilize even MORE volume. The test platform he made, he bolted to his 350Z and he loved the results and was talking and hyping this thing up like none other... He couldn't keep the hood of his 350Z on while testing this, and then when it came time to try it for the FX he found out that the difference in engine mounting and firewall placement made it a no-go for the FX. The firewall wouldn't allow it.
So go with the 1/2 for an FX