I would use the fiberglass rope you see used in stove seals. It needs to be clamped down tight. You can check for air leaks by making up some soapy water in a spray bottle and shooting air into the tank and spraying the water around your welds and seals. If you see bubbles coming out then you have an air leak.
Also for that 3/4" hole you could just cut it out and weld in a 1" coupler.
If you can find a 3/4" pipe and 1" pipe that will nest into each other tightly, that would make for a very thick nozzle. Drill out about four or five 3/8" holes from the very center.
I welded my flute nozzle in directly and welded threaded tips on each end to cap them off. With your unit you may have to do the same, since it looks difficult to reach your arm all the way in to screw in a long pipe.
For flow it’s best to have the pipe open ended on each end and have it going all the way across the tank body.
Edit: Here’s how I had my updraft built. Ran like a scalded dog until I got hit by a minivan and flipped it.
Bear in mind mine was made for a 2 liter engine but the tall ammo cans are great, I just had a bag hose clamped to the pipe welded into the lid.