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 - Donna
'09 Kawasaki KLR650
'98 Rhygin Juke SS / '12 Gunnar Rockhound 29 / '16 REEB Dikyelous
"Our task must be to free ourselves from this prison by widening our circle of compassion to embrace all living creatures and the whole of nature in its beauty. Nobody is able to achieve this completely, but the striving for such achievement is in itself a part of the liberation and a foundation for inner security. " --Albert Einstein