I began this project over a year ago after reading the numerous threads on the topic. I tried the template method, but I could not get the tank to fit in my '89 Firebird Formula. The problem was that the bottom of the tank was hitting the bottom edge of the fender.
Camaros have different fender curvature than Firebirds, and all of the existing tutorials were for Camaros. After ruining one 4th gen tank with excessive heat gun application trying to get it to fit, I realized that I was going to have to take a different approach.
I mounted the bottom of the 4th gen battery tray flush with the subframe, about 1" higher than it was from the factory. To do this, I had to weld a new panel over the factory battery tray (which was already rusted out), which I cut out.
The first picture is where I cut out the bad sections of the old tray.

 

Here's the tank installed. I had to use some all-thread rods to hold the battery down because the retaining edge on the 4th gen tank was cracked off, and I wouldn't be able to use the factory "puck" retainer. I spent a day going to all the salvage yards in my area trying to find the best tank to use, and every single one I found had that retaining edge broken off, which leads me to believe that it wasnt very strong to begin with.

 

Here's how the tank looks from the bottom. I still can't fit the factory fender support brace right now; I'll have to fab something up here soon.
As it is now, the tank still extends below the bottom edge of the fender by an inch or so.
So there you have it guys. Hope this can help someone out.

 

Looks good