just replace the slamvan3.yft, slamvan3.ytd, slamvan3_hi.yft in your mods folder through openiv 

path: 
OpenIV/GTAV/mods/update/x64/dlcpacks/mplowrider2/dlc.rpf/x64/levels/gta5/vehicles/mplowrider2vehicles.rpf

path for custom mods:
OpenIV/GTAV/mods/update/x64/dlcpacks/mplowrider2/dlc.rpf/x64/levels/mplowrider2/vehiclemods/slamvan3_mods.rpf