310.16 Is Based on Ambient Temperature of 30?C (86?F)
The correction for ambient temperatures go up to 80?C and I think that?s around 175?F with a correction of -41% of the table ampacity.
IMO the mechanical code will cover this. The clearance to combustibles with a single wall vent is 6" & clearance to double wall is 1". I hope this helps. Ron