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Now if you decide to max the upgrades out in the trainer i recommend setting the interior style to stock because any other style will cause your game to bug

So I was able to use it and the upgrades I recommend using simple trainer https://www.gta5-mods.com/scripts/simple-trainer-for-gtav
when you use it go to vehicle spawn then search vehicle type in primo and choose the first one names Albany primo custom Shelby if it spawns the other Shelby you installed it wrong but if it does spawn you should be able to mod it to its full extent through Bennys garage in the vehicle options section in the trainer