Installing Coal Furnace Into Existing Plenum

Hi, my name is Tim and I recently made the jump from oil forced air to coal however I have some issues hooking my furnace up one issue is that I would like to hook the plenum into my existing oil furnace plenum, not that big of a deal I just want to know if anyone else has done that and how you did it and how well it works. The other is hooking it into the chimney, as I said I have a oil burning furnace which is staying right where it is. Due to the placement of the coal furnace relative to the oil furnace in order to utilize the existing plenum I have some issues in hooking into my existing chimney. One is I could go straight from my furnace into the bottom part of my chimney near the clean out hole, does not seem like that great of an idea, the other is I could run a 90 off from the back of the furnace and straight up and another 90 into the chimney, again doesn't seem like a great idea. The other is I could possibly run a 45 of from the chimney and another 90 to get into the chimney. As you can see I have no clue any suggestions are greatly appreciated,