Lessee - if it was making 235HP and was flying at 55MPH - assuming a 90% efficiency on the prop -
[(550 ft-lbs/sec)/HP]*235 HP / [55 MPH*(5280 ft/mile)/(3600seconds/hour)] * .9 -> 1400 pounds of thrust.
If it had a full fuel load of 150 gallons and a 200 pound pilot that would put the plane at roughly 900 pounds of fuel, 200 pounds of pilot, 1500 pounds for plane -> 2600 pounds.
Not a clean plane, so if it had L/D at 10:1 then 260 pounds of thrust went into holding the plane up and nearly 1100 pounds went into pulling the banner, almost 4 times the normal drag.
Any better numbers and I'll run the above to compensate.