Well, I don't have much, because the weather hasn't been all that great. But I figured I would update with what I have.
Remember, this is for my '73 240Z that's 31 years old.
I used 1X2" tubing with a 1/8" wall thickness. I bought 8 feet of it. With the spare tire well removed and laying underneath the car, I found the center of the space the fuel tank is going in. Between the frame rails, the center is 14 1/8" at the front and 19 9/16" at the rear. I will have a picture of this measurement that I will post in the future.
IMPORTANT:
I offset the tank to the passengers side 1", because the design of the tank tapers in towards the back on the passenger side a bit. I figured this will make the weight of the tank more balanced in the chassis.
The drivers side rail that the tank attaches to was cut at 27 3/16". The passenger side rail was cut 26 7/8" (DO NOT use these measurements for your car! I found, after installing my roll cage, that my car was hit in the rear on the passenger side, and it's pushed in a hair over 1/4") The distance between the two rails on the tank is 17" from center to center. The center of the fuel tank is 10 11/16". Look at the link to the picture to see how the center of the fuel tank was measured (from those little "L" things).
The LS1 fuel tank has a regulator on the return line, so I gutted it. This was done because the GN engine has a fuel pressure regulator on the fuel rail.
Pictures:
Fuel tank rails
Measuring the center of the fuel tank
Fuel module
Better picture of fuel pressure regulator
Regulator internals
I will have more updates of the finished rails along with more measurements to cut and weld your own.
!M!