Hugo, Maybe you already have your answer...but if not, maybe I can help.
All swift crews were extensively cross trained in the battle positions on board swift boats. As crew Radioman, I was in charge of the communications, as well as other activities...including gunner on mount 51...the twin fifty's fired from the guntub.

The gun tub is slightly wider at the top and tapers down slightly, maybe five or six inches, and then goes straight down. There is an opening in the control cabin that allows access to the firing position.

The twin fifties are mounted on a frame that is attached to the guntub, and pointed forward. The fifty cal rounds feed from either side, that is, on the left hand side for the left hand gun, and the right hand side of the right hand gun. Facing forward, there is an aluminum container approximately 7" wide and two and half feet long, and about twelve inches deep on both sides of the tub. The top of this ammo "can" was about thigh height. Belts of ammo were connected together so that there were several hundred rounds available for each gun. There was not a seat, or stool in the guntub I operated from. Rotation of the tub required shifting of your body weight. Accurate fire required that you apply your body weight against the fifties as they fired to control drift...even though they were counter opposed, they still had to be man handled to stay on target....not that hard to do, but necessary for me. Other combat vets may have used other techniques...I was 19 years old and weighed 130 pounds, so my experience may be different from someone who was heavier than me.

When mount 51 was firing, the hot brass dropped down into the cabin, and sometimes onto us....and it felt and sounded like we were in a base drum that was being pounded. I, as well as many others, I'm sure had damaged hearing resulting from the concussion. I hope this helps, and good luck with your project. Neal Stone