the Carry Select section on a decimal based computer is the section that when when a carry needs to happen, allows for the carry impulse to pass to the receiving location. this "mux" must allow for the initial counting pulse to be selected during a transfer, and then later based on the Carry Block signals to allow the pulses to pass. The OR gate i was using to combine these signals didn't seem to have enough drive strength to properly drive the receiving location logic properly due to the length between them. so on this revision I've added a Schmitt trigger to the design, which seems to solve the issue with both drive strength and line noise.

here is the schematic: