Anyone look at this venerable favorite?
http://www.amain.com/Futaba-S9650-Se...tal-Mini/p9495
Wow...those mounting holes are 43mm on center...guess what the chassis holes are?
43mm exactly!
http://www.servocity.com/html/s9650_...l#.VDqCARE0mL4
Whether or not the horn offset works is something to be investigated, but this thing should bolt in with no servo mounts required - they are built into the servo. Just measured the horn offset, it's within 1mm of dead center.
This might be good money if you're a Futaba guy.