I thought I'd hop back in here briefly and offer some new info I personally was involved in gathering today, specifically on that "gap" between the front tank bracket and the frame rail.
I helped my buddy KCMongo collect and take back to his shop an AD 1/2 ton chassis with a frame mounted tank on Wednesday of this week that a local fellow was going to scrap. Today, while at KCMongo's I helped him remove some of the reusable pieces off that chassis, including the frame mounted tank and brackets/straps.
When we removed the single bolt through the frame that threads into the forward bracket, low and behold, we found 3 approximately 1/16" thick body style flat shims that filled the gap between the frame rail inner face and the bracket.
The shims looked something like the rough drawing below and the tank and brackets did not appear to have ever been disturbed, so I'm pretty confident they would have been factory pieces used to fill that gap you are/were experiencing when mounting your front tank bracket.
Anyhow, just posting this in here for when other 'Bolters come across the same predicament and it may help them resolve the issue.
