I have never personally cut open a propane tank. If there was ever any doubt tho, the best choice would be to check out the local weld shops or mobile welders.
The thing is, if you are not 100% confident in what you are doing, there is a chance you could forget a key step in the plan and thats when bad things happen.
Just sayin!!!!!
Now I have cut a diesel tank, I used dawn dish soap and filled with water.
There are so so many options, but the one you do NOT want to do is use the exhaust off the car.
Newer cars are emissions happy and are not plentiful with carbon monoxide, and will not displace the oxygen. There for causing the ALL MIGHTY Ka BOOM!!!!!!
And please don't worry about having a stick welder. Its not the always whats in the box, its who's holding the stinger. Much of my work is done with the stick welder.
Even if you only have an AC machine, it will more then build a fine smoker.
Many rods are built specific to run on AC.