yeah its very hard to get these flares perfect, and let me tell you!! they MUST be PERFECT for them not to leak...i ended up going out and buying a $70 dollar double flare tool instead of the cheap pos $30 dollar one i got from autozone.. then i did a little research on how to make a good flare and with the new tool i was able to make some really nice flares that dont leak, i did screw up one flare though slightly and wat do ya know it leaks so i have to redo that one.

 

Did you ever finish this up? I was hoping to see pictures of the completed setup.

 

In the interest of those searching in the future...
The mounting studs on the FC booster are 2" long, not 5".
For some reason, the outside (as it sits in the car) studs are slightly longer than the other two, but I can't see any reason why, or why the same size for all four would be a problem.

If you're experimenting with different boosters, don't limit your search to just Mazdas.
There are but a few brake booster manufacturers and all the car makers use them, so interchangeability is pretty common. A few of the higher end Hondas have a double walled firewall and their boosters have longer studs to suit. I haven't looked very hard but I'm sure there are others.
I just snagged one from a '97 Integra but have yet to begin adapting it.