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.
As soon as the temp hits zero, everyone needs to go out and try to start their coach with low batteries and no engine heater. Use lots and lots of ether.
My mechanical Cat 3406B would start well in sub freezing weather. It would turn over several times then start coming up to speed and do what we call pop corning-stamering and missing until enough heat built up to start running consistantly on all 6. Usually would take about 2 minutes of pop corning to come up to speed. Good luck, TomC