Welcome to the forum!
The 40 gallon tank would make a great reactor body for an updraft charcoal gasifier. However, I suggest a Flute Nozzle like Kristijan Leitinger has made, also Steve Bowman and his Charcoal powered Toyota Corolla. I’ve also used a flute nozzle in my Mazda b2000. They don’t melt away like Gary Gilmore’s original nozzle design.
The flute nozzle has the benefit of entering the nozzle from the side, but the holes point directly up, keeping the heat above the nozzle. It also allows you to add water drip in from the air entrance, the heat that the nozzle absorbs will vaporize the water into steam and flow up into the reaction to make hydrogen and have the added benefit of keeping the nozzle cool. Kristijan suggests using some thick pipe, for my Mazda I used 2.625" OD .625" Wall DOM tubing I got on eBay and then I welded some 2" NPT threaded ends so I could cap it off for shutdown.
For water drip I had a 1 foot long stainless steel straw attached to a rubber hose and I simply placed the steel straw in the flute, and used a valve to meter the water to about one drip per second. You could use a bucket or plastic tank to hold the water unless you want to strap the water tank to the gasifier body and then I’d suggest something metal, maybe a paint can or fire extinguisher. I wouldn’t suggest anything other than water, waste oil makes a smoky flaming mess.
One other thing to consider is some form of flame arrestor for the nozzle, I simply stuffed a copper choreboy pan scrubber and it kept hot embers and flames from shooting out on deceleration and shutdown without restricting airflow.
What is the diameter of the pressure paint cannister? You don’t want it to be too small, because you will need to check on the nozzle every now and then to scrape slag off of the top. The slag will form around the holes of the nozzles and make volcanoes, but can also clog them at times especially if you forgot to remove any nails. You could also just weld in a 2" or larger coupler that you keep plugged except for maintenance for scraping and inspecting the flute.
For a gas exit I had it exit at the very top, and clamped a rolled up a tube of diamond grate expanded sheet metal onto a 2" pipe. This acts as the roughest sort of filter and keeps the big stuff out, and then I used a bag filter in a tall ammo can. The soot catches on the bag and will filter better as it’s covered in the layer of soot, but you should shake it clean every now and then depending on your mileage/hours.
It’s suggested to add a thermometer at your gas exit, once the gas comes out hotter than normal it’s time to top off the system.
For fuel size and best results, you want to crush down the charcoal in between 1/8" to 3/4" in size, you can screen the charcoal with different sizes of expanded sheet metal. I like to pre screen my charcoal after it’s cooled from the biggest size, and only crush what didn’t pass through that screen. I’ll screen it again at the minimum size to get the dust and tiniest pieces out after crushing. Please wear some kind of mask when processing charcoal, ashes and dust will irritate your lungs.