Hi Bob
Trying to picture the concept from reading a description is always difficult as there is a lot of room for interpretation. As Danny mentions above a drawing of what you are trying to achieve would be very helpful. But from what you have written above I have the following observations/comments/questions
What you have described would make a cracking cold smoker however I think it may turn out to be rather inefficient for hot smoking. The efficiency is all about balancing the fire box size to the cooking chamber, the chimney volume to match the fire box and getting the correct opening size between the firebox and the cooking chamber. Many of the old timers on here will usually do this by eye however for us mere mortals there have been some very helpful interactive pit size calculators developed to help us out. I have put links to 2 of them here and you may want to run your pit dimensions through them before you begin.
Feldon Central
Smoker Builder
Using the typical 55 US Gallon drum dimensions I have put some figures through based upon your comments above. I have also made some assumptions regarding your fire box size to give you the recommended volume.
Drum size - 23" diameter x 35" length
Firebox size - 24" long x 20" wide x 12" high. (It is the volume that is important here so the actual dimensions can vary)
Chimney diameter - 2" (You said that you were going to use the existing drum filler hole and these are usually 2" diameter)
Firebox to cooking chamber opening - 3" diameter
When you put these through the calculator a couple of things immediately jump out at you.
- With such a small 2" diameter chimney it will need to be just over 76" (6.3 feet) long
- The recommended Firebox to cooking chamber opening diameter is 7.6" - which is a little over twice the diameter of your proposed 3" tube
Now these figures will change slightly if you tweak the volume ratios between the smoke chamber and the firebox however the ratio above would generally be considered as being about right.
The efficiency of the firebox to cook chamber opening would also be further reduced if you were to have a length of the 3" pipe between the fire box and cooking chamber rather than have them bolted directly together. Having the cooking chamber closer to the fire box though may need to reconsider making the "manger" out of wood - angle iron may be safer.
For the fire box you could go back to where you got the Castrol drum from and see whether they have another smaller drum that you could bolt directly on.
I think you are off to a great start so please do not take the comments above as negative. It is just that I would hate for so much enthusiasm to be quenched by your first build having a few avoidable design limitations.
Cheers
Wade