New Correspondence Framework - MT300 Customized Creation

I am in the middle of an implementation using the new correspondence framework, however I am having issues with creating a customized MT300 to send to Misys.   I have successfully been able to generate the SAP provided MT300 on outgoing trades.   However when I copy MT300 and begin to make field modifications in SM34 - TCORFVC_MAPPING, my trade creation short dumps when attempting to save.   I am trying to add in the 83a record for Fund / Beneficiary.  

DIRECTION     FORM      POSITION      PATH OF SEQ   FIELD TGT SOURCE    OPERATION    PREFIX   SOURCE MAP   PARAMTETER OF MAP

Outgoing          Z_MT300 83                    A                         83a                                 Move                                CONST               /NAME/COCD            

Anyone have any experience with MT300 custom messages and may be able to share insight?   I am not finding a ton of documentation when it comes to the customization of this output.

Thanks!