The ft232rl is a usb to ttl serial adaptor. It can work at multiple voltages based on its vccio pin. If your adaptor doesn't have a selection jumper, or you can't/won't rewire it, then you have two options. Build a level shifter, or buy one.
A voltage divider will work fine for the ft232 tx to bbb rx. Inputs are low impedence signals. You arnt using it to drive a load. For bbb tx to ft232 rx, its a bit more difficult. It depends on what the ft232rl's minimum for a Vin logic high is. If its 0.7VCC (0.7 * 5v = 3.5v) the BBB tx will most likely not work. If its 0.6VCC (3v) or TTL level high 1.8~2.2V, the BBB tx will work as is. Otherwise you need a level shifter.
There are some transistor non inverting level shifter designs as well. Or use a dedicated level shifting ic. You only need 2 single direction lines after all for basic tx/rx shifting.
This site has a bunch of questions on 5v to 3v or 3v to 5v level shifting that can apply to your needs.