I would contact LMC truck. Post back what they tell you, I might be getting some LMC panels in the future.
"PPG D74LF" Is this a misprint? Did you mean PPG DP74LF, with actvator?
I have the PPG data sheet on DPLF. and you should too.
If you cannot find out what is on the panel, I would sand it to bare metal, and then prime it with the DP74LF. Optional, use the DX1791/DX1792 wash primer, under the DPLF.