Futaba 149 DP as Dual receivers
I have several GS models with Futaba's 149 PCM RX's installed in tandem configuration. It sounds like you covered all the bases.
Have you looked at Futaba's web-site for suggestions on the expected range? Do you have one section of the antenna mast extended?
With any model especially gasser's, I suggest an on the ground engine running at idle and then mid-throttle range test. If you have a substantial difference in range you may have a problem that's generating interference.