What are the problems with LiFePo4 float charging? How should I charge them?
LiFePO4 batteries have a very low internal impedance so constant voltage charging does not work well. Essentially you provide the battery with current and it controls the voltage.

You do not say much about your application but these batteries will give maximum life when they are kept in the 50-80% charged range so float charging is a bad idea. Given that they show very small changes in voltage with charge level it will not be easy to do this simply.

If you look at your datasheets there should be a charge curve. This will show that at around 90% charge the voltage starts to rise quickly. If you design your charger to pick up the voltage at this point and then to shut off it will give the easiest approximation to this.

The big question is when to turn the charger back on. This will depend on your application but my best suggestion without getting into Amp Hour measurement is to use a delay which you can set up to suit your application and the battery capacity.