2.5 to 3 minutes is about normal and yes you can convert it to a tank. remove the tank from the motor and remove the plastic pick up tibe that is connected to the nipple on the backplate. Drill a hole through the old tank that matches the size tubing that your going to use[ small preferred,] make sure that you deburr and chamfer the hole. feed the new fuel tubing through the hole in the tank and hook it up to the fuel nipple.Reassemble the motor ,make sure that you don't kink the fuel tubing. Connect the other end of the tubing to your external tank.
I usually find an old Baby Bee tank to do this to and mount that to the black widow. It save ruining a very expensive part.
You really shoulden't go over 1 oz as that will be quite a good run. Make sure that your tank is close to the motor as there is not a lot of fuel draw.
I've done this for at least 40 years so it does work.