Some Honda 450s had LT a-arms with shocks in the 19"+ range, some even had 20.5". I can't say I have heard of YFZs running a 19" shock. What a-arms did you track down and/or buy?
Even if the shock measures the correct length, doesn't mean it is valved for those A-arms. ATV-Fourplay runs a different leverage ratio than JB, Walsh, Six5, etc.
If you didn't buy a-arms yet (although it sounds like you did): why not consider getting set of Houser/JB/Walsh A-arms and having the shocks revalved/rebuilt for those a-arms?
Is it the PEP/LSR combo in your signature? I'd just send the shocks to Baldwin and have them revalved for those a-arms. If they need to be shortened, he will do it.
2009 YFZ450R
Baldwin, DID, DP, Houser, JB, Maxxis, PD, PCV, PEP, LSR, Rath, RPM, Sparks, SSI, SunStar, Works Connection
Special thanks to Baldwin Motorsports, PEP, SSI Decals, & my checkbook!!