You are looking to update Sage 300 Accpac to version 6.3 and would like to know if there is currently an AvaTax connector that will work with this version of Accpac.
Sage 300 - Accpac
At this time there is no AvaTax connector for Accpac 6.3.
As of March 2016, our Development Team is currently working on releasing a version of AvaTax that will work for Sage 300 - Accpac 2016 v 6.3
It should be available by the end of June 2016.
Please review your Admin Console > Home > Get Started > 3.Download and Install Integration Software page to see if the connector for Sage 300 - Accpac version 6.3 (2016) has been released.