Get a Maximum Motorsports K-member.They are the best HD K-member made and to support the weight from the Fat Boy you will need that strength to be safe.
You will also want to add a strut tower support and maybe an X brace for the K-member.
I would also go with some coil over struts so you can select some springs to support it and easily adjust your ride height.
I have thought about doing something along those lines myself as I have several 429/460 engines and a C6 or two sitting in my garage, but my injuries have kept me from doing much of anything for the last few years, so it may be an un-achievable goal for me at this point in my life.
A 460 would be cheaper no doubt, but an aftermarket 351W block can be bored and stroked to at least almost as many cubes nowadays, then add a turbo or two and you can easily reach over 1000 HP. I intend to see just how far I can push the stock block 5.0 I have, then go with a Boss 351W block, bored and stroked to 427/428 cubes eventually and turn up the boost on it and see if I can reach the 1000 HP goal in a street driven Fox.