Installing a Boyd's welding gas tank

[Oldtruckwelder]

I would like to do a shout out for Boyd's Welding , a really well built and Quality tank IMO.. Also their customer service was very prompt with dealing with a small problem with getting banged around during shipping.
Currently installing in a 1957 stepside and it fits wonderfully between the rails.There is a little bit of wiggle room from front to back, QUESTION--- Is there any advantage to installing more forward or back before I nail it down.
Thanks
JT from SC