We never used stove pipe & I don't think I'd want to. We always used 3 or 4" pipe or square tubing with a 90 elbow on the end of it. You only need enough pipe to direct the heat and prevent any actual flame coming out the end - 4 feet is more than enough.
Not if it is to short to reach the motor....
....not if it's your first time...LOL
Stove pipe works extremely well for wood stoves and it is not as much $$ as 4 inch Schd. 40 and elbows....just saying.
I have used air duct in a pinch. at -40/-60 you use what you got.
back in the mid/early 70's you would not want to be in the Yukon/NW Territory w/o a set-up to start your car.
FWIW I agree....12 feet is way more than you'd need, but I assure you the heat will get to the engine bay, AND your bus will not burn to the ground.
All I wanted to express is Safer is better than sorry.
PS It beats lugging around 40 lbs. of pipe....I guess exhaust tubing would work too.