You might be over-thinking this one.......It shouldn't make any difference what gear it is in, especially if you have a good functioning emergency brake applied.
For the record though, having it in first gear, which will give you the greatest torque multiplication, would make it easiest for you to move the truck/over come the e brake by turning the crank bolt with a wrench. Same for reverse. So the higher gears would lock it up better for what you're doing.
Again, unless your bolt is super stuck or the ebrake doesn't work, it really shouldn't make any difference.
On engines that rotate counter clockwise when viewed from the front, I used to put a socket on the crank bolt with a long cheater pipe that would wedge up against the frame. Turn the crank with the wrench first to make sure the cheater has no slack and is tight up against the frame. Then hit the key and just bump the starter, this will usually break the bolt loose with one bump. Be sure and pull the coil wire first though to ensure the motor doesnt start.
Randy Rosetta KDØKWX TLCA #19351 69 FJ40 "Meanie" & 1970 Boss 302 Mustang