You might also want to switch to an "electronic," rather than the conventional "thermal," flasher.
The resistance of the LED's is vastly different from the incandescent bulbs. Since the common "thermal" flasher depends upon bulb resistance to determine flash rate, adding lots of lights (like hooking up a trailer) or changing the total lighting resistance (like installing LEDs) is going to SERIOUSLY screw with the flash rate.
Electronic flashers have the flash rate determined internally, and therefore don't care how many lights are connected (as long as you don't exceed the number of bulbs or "sink current" for the unit.)
Expect to pay ~$10-12 per unit (you'll want two,) and ask for an equivalent to a #552 thermal unit...
5-90