I am doing something similar. Using Arduino Shield with direct Serial to EmonCMS. I asked the question, & basically you will need to recompile the TX latest software to reflect the possible change of processor (Board setting), turn off the temperature compilation which I think is the default, then change the LED pin which is different on the shield. The communication should apparently work without change.
Please note I am in the process too, so as yet untried.
Look here for the discussion:
EmonTxV3 Continuous Monitoring Firmware (v1.0-beta)
Good Luck! Doug