If you want to fit the bike like a road bike, get the top tube that fits like a road bike. I really dug the Straggler geometry since I could get a top tube length closer to that of my mountain bikes and get by with a shorter stem to make up the reach. Longer top tube + shorter stem means more weight centered between the wheels. Think about a lot of the modern all-mountain and trail bike geometry, looooong top tubes and short stems paired with wide bars to make up the reach. Wide bars arent an option really beyond 46cm Woodchippers or Luxy bars so Surly extended the top tube out.
If you want the bike to feel like your Troll, get a similar length top tube to the Troll (looks like the 56 or 58 might even be a better fit). Measure from the center of the steertube to the inside edge of the grip on your Troll. Make a triangle with one side being half the width of your handlebar, the next side being the effective stem length (not necessarily the printed stem length if your stem has rise), and the hypotenuse being the distance from the steertube to the inside grip of the handlebar. You'll want this hypotenuse to stay the same in order to keep your overall posture on the bike the same. Now make another triangle using half of your desired drop bar width as one side, the known hypotenuse, and stem length + bar reach as the third side. You'll need to use the Pythagorean theorem to solve for the stem length given that you know the other values (bar reach is fixed, bar width is fixed, hypotenuse length is known based on your current bike). a^2 + b^2 = c^2. If you're not good with math, feel free to post up the numbers and I'll run them for you to find the stem length that should fit best.
If you want the bike to feel like a road bike, go test ride some road bikes and make a note of what top tube and stem lengths work for you.