Nominal thickness for TO-263 (KTT) is 4.445mm. I have to presume that if the package thickness was not compliant with J-STD-020 and J-STD-033 that a MSL could not be assigned by the Package Group.
Please refer to : AN-2029 Handling & Process Recommendations
http://www.ti.com/lit/an/snoa550a/snoa550a.pdf
Answer to drying times is in : Table 5. Reference Conditions for Drying Mounted and Unmounted SMD Packages