Locating the fill neck was a critical operation.  When I installed the original tank many years ago I cut the hole in the gas tank cover to fit the tank.  This time I had to hit that hole dead on.  No grinding this time after it is painted.  Basically made a jig that located off of a common mounting hole that wrapped around the original filler neck.  Then using this jig I fit the new neck to the tank.  Not so easy to do because stainless is tough stuff to work with.
 

The locating jig off of the common mounting bolt hole.
With the new tank in tha car needed to verify that it would fit the cover.
Cutting the hole into the tank.  A real thrill into stainless and several angles.  Turned out nice though.
Fitting the neck to the multiple angles.  Another fun job of grinding.
To keep the neck long enough to fill the tank completely needed to make a sliver of steel to fill the gap.   I was able to miter the rest of the tubing so there was no gaps but this was a little too much to bridge....
A few minutes with the TIG and it is all sealed.