@bonino,
@Sonnar Brian has given you a far more OTT option than is likely necessary here. Any rangefinder that has been shipped from point A to point B has likely been knocked about a bit, so I'm more inclined to say that the RF in the FED is out instead of the lens not being to spec.
Infinity RF adjustment on a FED 2 is simple if you have the right screwdrivers. Remove the cover screw that's positioned under the FED engraving on the front (here shown under the F on this photo I shamelessly stole from Mike Eckman):
(Even if you have one of the earlier style of FED 2s, the screw will be in the same place. If you have the later FED-3-in-disguise with the shorter rangefinder base, the screw is on the other side of the lens mount, closer to the main viewfinder window.)
Hidden under that screw is the RF adjustment. You'll need a very small flat headed screwdriver. Set the lens to infinity, point the rangefinder at the moon, and turn the screw GENTLY AND SLOWLY until the RF images line up.
You should also check the close-focus; it's not easily knocked out, but it could be wrong. Set up a "test bed", with an easily-delineated and contrasty object placed vertically and well-lit. Set the FED 2 so that it's 1m from that object
to the film plane (not the front of the camera!), and make sure that the film plane is perfectly parallel with the object. If the distance scale on the lens says 1m when the rangefinder images match up, then you're good to go. If not, you have to remove the lens and GENTLY AND SLOWLY rotate the sled-shaped cam follower at the top of the lens mount, then retest the focus.
Note that once the 1m setting is correct, you'll have thrown the infinity out again. And, once you reset the infinity calibration, you'll have to recheck the 1m mark. With each adjustment you get closer to it being in spec; hopefully you won't have to do much, but the first FED 2 I owned was wildly out at both ends and it took hours. Soviet LTM cameras I've bought since either didn't need the 1m adjusting at all or it only needed tweaking slightly.
At any rate, as you don't have any "known good" Soviet rangefinder bodies or lenses at this point, you should do this and test the combination with film before you jump to modifying
anything with shims. Once you have a lens/camera combo that you know focuses reliably, use that as a baseline to test anything else you add to the kit.