TTL voltage shift from 5 to 3.3 Volts "UART"
The 2.4 V is the minimum for a high level TTL output. A TTL input needs at least 2.0 V, to give you a 400 mV noise margin. But that's for TTL devices, or TTL compatible, like HCT-CMOS. An HCMOS device will typically need 0.7 Vdd as minimum for a high level. For a 5 V supply that's 3.5 V, so you probably won't make it. You can use an HCT single gate IC as level shifter.  

The resistor divider probably won't cause much problems, except for very high speeds. Since this is less than 100 kbps UART, chip resistors shouldn't be a problem.