I've started. I'm using standard servos in both the rudder, flaps and ailerons. I am going to have to cut away a bit of the tabs or the L-brackets to get them to fit though. It will be tight. I don't mind having to do that.
My issue is that I am having to dig for screws (haven't found any yet) to mount the servos to the L-bracket mounts. I'll find them - or just order a viper 2M to build instead.
There is also an old manual that you can check on.
http://fbjets.com/manual/L39.pdf
Aileron, flaps and rudder install is the same as in the manual.