A water based flame thrower maxes out around 125 psi anyway. More pressure than that, it will atomized more quickly and have less range. That is for a 1/8" - 1/4" nozzle. Haven't tried a larger nozzle. Max range in the 25 foot category.
You run with 50:50 regular drywall mud and water mix that 25 foot range increases to 25 yards with the same setup but at 200 psi which that tank probably isn't rated for.
My set up was the old stainless steel water fire extinguishers. Complete with siphon tube, rechargable by screwing off the nozzle, Air charged through a car tire nipple fitting to 200 psi. Sucker weighed 30 pounds and wound up residing in a tank. Saw two on e bay 3 days ago. $20
2 years ago my local fire extinguisher vendor has 20 of them for $15 each. If he didn't sell them, he was going to get rid of them for scrap. Probably gone now. but it wouldn't hurt to check around. Fire extinguishers are charged at 200 psi unless it is a CO2 extinguisher. You might pick up one of them that has had it recertification life expire. You could comfortably run 200 psi in a 1500 psi expired extinguisher.
I assume this if for a tank rather than a portable grunt carried flame thrower. That sucker would get heavy. 9 pounds per gallon of water.
Now you would have to invert that tank or install some kind of siphon tube to feed the simulated napalm.