That would probably work. The rpi operates at 3.3v so in a perfect world your R2:R1 ratio would be 2:1 to get 3.3v from 5, so if R1=10k then R2=20K.
You only need to do this from the RAMBo’s TX pin that’s connected to the rpi’s RX pin. @jeffeb3 suggested above that you should not need to step up the rpi’s TX to 5v for the RAMBo. X39’s ground pin should be fine.
https://ohmslawcalculator.com/voltage-divider-calculator
[EDIT]
To clarify, in your schematic instead of Vcc 5v you would connect to RAMBo’s TX pin (5v), and the 3v output goes to rpi’s RX pin. The RAMBo’s RX pin can be connected directly to the rpi’s TX pin