How far off are you now? You can fix it by adding length electrically and it might be pretty easy, modify the transmission line between the NMO adapter and the antenna or maybe add some inductance at the base. I was wondering how it would go tuning with the dual SO239/PL259 discontinuities at each end of the feedline.
'91 Pickup - Imelda
'08 Tacoma TRD
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29
"To live is to suffer, to survive is to find some meaning in the suffering." -- Friedrich Nietzsche