How to setup PCB to correct for signal integrity issues after boards are made?
Add a pair of resistors to the end of the lines. One up to VCC and the other to ground. This will allow you to terminate the line at the end in addition to (or instead of) source termination.

The problem with source termination is it works best with only one endpoint and you have many. The intermediate points will see a step on the edges, the placement of which will depend on the relative impedance of the line to the source resistor. 

Parallel termination will allow you to reduce the reflections which works better with lots of loads. Problem is you need the pair of resistors because the driver may not be able drive the desired resistance to ground or VCC. So the pair of resistors can be chosen to provide the desired impedance pulled to some intermediate voltage which the driver can handle.