To install you need OpenIV, then just open vehicles.img and make sure you are on edit mode and then drag and drop the files in there.
Then open handling.dat and find Packer and change the line to:

PACKER 8000.0 0.40 80 0.0 0.0 -1.5 0.0 5 0.137 0.4 119.0 0.18 0.54 0.15 40.0 1.25 1.05 15.0 0.15 0.4800 1.4 1.2 1.2 0.14 -0.13 0.0 0.60 1.0 0.6 1.2 1.5 0.0 40000 8 300000 2

Enjoy.

Credits

Amistech Games- for making the truck
GTA Mods Del Norte- for porting the truck over to SA
CarCrasher730- for porting the truck from SA to IV

Changelog:

0.1
Fixed the colors

0.2
Removed unused vehicle reflection texture
Fixed door collisions