With this weapon option becoming more and more popular, especially with the new Grey Knights codex (S8!), TL autocannons are in demand. There is a relatively simple way to convert them through the aegis defense set from GW, but those run at 20 a pop. If you're running three Dreadnoughts, that can get kinda pricey. This method takes a bit longer, but you'll save some cash.
The easiest version of the dreadnought to convert is the venerable. AoBR guys can be used as well (hey they're cheap on ebay), but they are a bit more difficult to work with. What you're going to do is cut down the arm sockets to symmetrical shapes.You'll have to cut down the pieces a significant amount to get them to this size.
On the opposite side of the newly created sponson, drill a small hole and place a magnet inside.
http://www.kjmagnetics.com/ is a good source for magnets.
Cut off the plastic pin on the dreadnought arm and place another magnet. Make sure you test the polarization of the magnets before you glue them.
Now you will need plastic tubing for the barrels.
Using the smaller of the two, determine how long you want your barrels to be. Place the tube in the sponson and mark with a knife where you want it to cut off.
Glue your piece in the sponson. Do this one by one, so you can gauge the length of the barrels off of your previous barrels.
Next, take the large bit of plastic tubing for the ammo cartridge.
Now all of the holes/cuts from cutting down the sponson need to be covered. Instead of going out and buying plasticard, take the plastic from any package you have lying around.
Using your hobby knife, cut this to size for the sides of your sponson and glue them (they can be a bit larger and then be trimmed once dry).
After you trim up the excess plastic, the rest is pretty much up to you. Go to your bits box and add stuff as you like. I took tubing/piping from a flamestorm cannon and tactical marine backpacks.
You can also add a small bit of tubing to the ends of the barrels so they don't look so plain.