A 26" wheel wont fit a Bighit. Only Betd.co.uk produces a conversion kit for 300$ or so. If you want a FR dually that resembles your FSR then the best solution is the Specialized SX frame, It is a beefed up enduro with a vanilla coil, it sounds just like the thing you want, the ride of your frame, coil shoch, durable for FR missions, plus its frame only so you can transfer your existing components over it!
Cheers.