Tex,
I would not leave a propane line in my airbox

If your going to do it? I would at least get a CFM+ intake manifold or drill and tap your stock one for a propane line.
Propane being feed into a big hot turbo compressing air sounds like a car bomb
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99 CTD DRW 4x4 Auto~ SLT Laramie quad cab~4:10's,
Edge Juice w/Attitude, MBRP 4" turboback, AFE Pro Dry S Stage 2, Airdog 150, Amsoil Bypass kit, Bilstein 5100's, HBS leveling kit, upgraded IC boots, BFG commercial T/A traction's,