1000.2 arm length

Why do you believe that you have the wrong arm? The arm for the .2 is permanently attached to the backing structure which will only bolt onto the 1000.2 reflector. The .4 has as detachable arm. The yoke for the LNB for the .2 and .4 look similar but the alignment holes are arranged longitudinally on the .4 and transversely on the .2 so they wont interchange. The arm length for both dishes does not change whether they are pointed to eastern arc or western arc but the .4 is a little longer than the .2 in either case. I would go out to my truck and measure them for you but it is -6F outside and I don't feel like going out to do that as it will not solve your issue anyway.