I carry a 9mm pistol, but have a locking gun box in my SUV that holds a S&W .357 magnum revolver when I am in the outback. The revolver is with a holster. I have two magazines for the pistol and two moonclips for the revolver.
I also do not see a problem with two different loadings for your purposes. If I breakdown in the outback and have to hoof it, I prefer the .357 magnum, 5" barrel N frame S&W. In this case the 9mm pistol, with two magazines, is my BUG. I feel more comfortable with the .357 magnum when in the outback.
I sounds to me as though your scenario is not the asphalt jungle, but rather the wooded range. For me the .357 magnum is head and shoulders above either a 9mm, or .45acp pistol. If I were to take a pistol under those conditions as a 'Truck' gun, then I would make it a Desert Eagle in .44 magnum. However, in California a loaded gun in a vehicle is a CCW permit situation. So the Desert Eagle is not applicable for me.
Good luck in your choice. I just know up in the Sierra Nevada mountains I'd be more comfortable with a GP100 than a .45acp.
On the other hand, have you given thought to that Marlin 1894C as your 'truck gun' for trips out back ? Same ammo as you SP-101, sling it and easy to carry if you have to abandon the truck, more accurate and more energy ballistics (can match a 30-30 inside 125 yards) than the GP100.