I've seen a guy put gravel in there to get any tough stuff out, then he put it on a spinning device to toss the gravel around. 

I have successfully washed them out with soap and water then let set to dry in a warm place until I could not smell the fresh gas odor at all. I have hurried the drying process by putting the wet tank on the grill with low heat. Note that the  grill was outside in the middle of a fairly large area with no one around (especially me). I figured that if it was toasty warm over a flame and did not go boom, I was OK. These were fairly small tanks like in a MG but I would do the same for a Mustang tank. If you are just fixing a leak, I recommend a new tank! There are some hidden things in closed areas like gas tanks to consider like if anyone ever used a leak fix coating or other additive in the gas tank. They have been known not to vaporize until reaching really high temps like welding. Then the tank fills with that vapor until reaching enough concentration to be explosive then BANG and it wasn't even the gas. We'll send flowers!

Another tank was my thinking , Tank was new about 4 years ago it is in a 71 mustang and I can pick up new on cheaper than blowing my self up. Car is close to paint and was thinking of EFI conversion if I get on the street. I watched a couple of videos of them dropping the pump in top of tank where the vent line exits. new pump would have vent and roll over valve built in. and there  is a hump in floor for clearance. Just thinking to cold to work in garage.

I used to clean motorcycle tanks with boiling hot water and Dawn dish detergent. Then rise with more hot water. The gravel thing really does work..

KM wrote:

Good point , on cold garage warming up. I would prefer not getting blown up once in lifetime is enough of that . And that was not a gas tank. it was more a question on drilling a four inch hole in top . I would probably go safe method new tank.

 
If all you are doing is drilling just fill it with water. Assuming the tank is in good shape and original it's better than the after market off shore stuff available.