-SWR is the least consideration you have. Your radios are not going to blow up and high SWR is not going to suck up the RF.
-Twin lead is the last choice for a conductor for us in vehicles, since you need to keep it perhaps 3-6 Inches away from metal.
-A Jpole is also less practical and really doesn't solve any particular problem and is needlessly complex.
-The basic solution is to approximate the 1/4 wavelength antennas that were originally installed. Use the existing coax, attach a piece of wire to the center conductor by twisting, crimping or solder, and ensure the coax shield/braid is making good contact with the metal vehicle body.
If a mount has been destroyed or the coax is short, punch a new hole, use a screw or bolt to fasten the braid to the body, and attach a wire to the center conductor. You can use a wood block or plastic item to attach and hold the vertical body. Bolt, screw or glue the block to the body.
Another method where you have excess existing coax, is to merely strip back the shield/braid, use the insulated center conductor as the antenna and again, screw/bolt the braid to the vehicle body.
Antennas don't have to be straight or perfectly vertical. Even a curled wire is better than none at all.
Your distance requirements are not difficult so you should have good success. Regarding antenna experimentation (and especially field expedient antennas) "If it's stupid and it works, it's not stupid".
Best Wishes, bill.