I take it your truck is a D250 SRW?
Before I say anything else I am going to be blunt, your trailer is way heavier than that truck was ever intended or built to pull. It is undersized and lacking in a lot of areas to handle that much weight. You probably should have bought a dually manual in a 2nd gen to be safe.
You are correct, a stock 92 feels like a dog compared to a big block gassers. No argument there. I had 2 440's along side the stock 91 and they would run away and hide from it. The difference was 6 mpg with the gas engines and 11-12 with the diesel.
You have 1 strike already with the auto trans. Even with the aux cooler you are going to be playing with the pedal to keep trans temp and engine temp from scaring the daylights out of you on long hard pulls. Along the lines of 240 degree coolant, 240 degree trans temp, 1200 degrees EGT's and 45 mph to keep things even close to sane. A lot is going to depend on where you tow also. Flat ground in lower altitudes will be a lot easier to maintain temps than playing in the rockies or Sierra Nevadas.
You want some more power but that doesn't come free. To use it effectively you need more air. To put the power on the ground you need more trans and deeper gears for leverage. If you want to keep going then we have some good ideas what has worked for us but its not going to be cheap and its not neccessarily going to be easy. You may have to make some choices about how much power you really want to balance the cost.
As an example, injectors will be a power upgrade. You can spend from $400 to $700 for nozzles and about $30 per to have the nozzles installed, pop tested and set. A good turbo that will handle the injector upgrade with more air can run anywhere from $1600-$2000.
The FAQ section has a lot of info in the top most threads about power and how to get it. Before you do any power upgrades your first step needs to be a full set of gauges so you have an idea if your changes go the right way. The first question you need to answer is "how fast do I want to go", then we can offer some options.
