Well, I am in the same boat as you... sort of. Right now I am trying to figure out how to safely get out the oil residue and fumes so I can make some cuts and sandblast it.
The 2 options we have at this moment is;
A: Fill it up full of soap and water and let it sit for a few days and drain it.
B: Use some gas to cut the oil and drain it and let it air out.
Whatever you do make sure its cleaned out real good before you cut on it or you will blow yourself to kingdom come!!! That being said, I have decided on keeping it vertical and going reverse flow. We are picking up a trailer kit from harbor freight this weekend since it is cheap and will work well for this project.