I have a 52 Chevy 1 ton with a 55 stock 235. My rear axle gears are 4.56.
I have towed our 18' flat trailer once around town to pick up some lumber, and I won't do it again. The trailer weighs between 3,000 and 4,000 lb empty. I could hardly get up to 4th gear. Even with the trailer empty it was to much.
I would limit yourself to towing about 2,000 lb with a stock 235. I have had a full 4' X 9' piled till it falls off, bed of mulch in my 1 ton, and that was about the limit on weight for the engine too.
A three speed is another thing that hurts you, as you don't have a low gear to get you moving.
I would recommend you get a bigger truck to take to the shows so you can tow both the 58 and the Mustang. Something with a Detroit Diesel, perferabily a 6-71 would be my choice for the job.
Grigg