In the late 1800's in northern Washington, a map maker was riding through the mountains when he came upon a prospector working at the edge of a creek. He asked the prospector if there was any gold in that creek. His answer became the name of the creek on the map makers map. Damfino. (it's a real place - Google it) :-)

I know that there is a mechanical drawing of the entire bottom of the car.  It shows the measurements of all of the mounting holes and everything you would need to put the entire bottom together.  I'm not at home so can't access my drawing but surely someone on here would have that also.  From this drawing you could literally make a jig to set up how the bottom would go together.  I'm going home for Christmas. If no one on here has a copy I could send it to you then.

I have this - don't know if it helps you.

Couldn't you take the 16 1/16" measurement on each of the rear rails and use that as the 'bottom leg' of your measurement
and
the 24 3/4" (I think) measurement on each of the front rails and use that as the 'top leg' of your measurement
and get your rocker panels parallel that way ?
(according to the drawings^^)
6sal6

6sally6 wrote:

Couldn't you take the 16 1/16" measurement on each of the rear rails and use that as the 'bottom leg' of your measurement
and
the 24 3/4" (I think) measurement on each of the front rails and use that as the 'top leg' of your measurement
and get your rocker panels parallel that way ?
(according to the drawings^^)
6sal6

It doesn't ,look like the rockers are parallel.  The front measurement mounting flange to mounting flange flange is 54-3/4 and its 55-1/2 at the rear.  The rockers seem to toe in 3/8" per side towards the center line.  Personally, I would not align frame rails based on bodywork anyway.  There's too much that can be tweaked or wavy in a mounting flange.  I would only use the locating points for the frame rails because the heavier gauge steel is going to be less prone to error.  If you start with a straight center line and ensure all your measurements are the same from center and square to the center line, that will be the most accurate.  If you need parallel lines off that center line its as simple as just measuring an equal distance at the front and rear of the center line and laying down a parallel line.  You can also always double check square with a 3,4,5 triangle, or any multiple thereof (i.e. 6,8,10 or 12,16,20).  Squares can be hard to use when the length of the lines are a lot longer than the square itself, but Pythagorus doesn't lie. 
 

John Ha this does help. I am using a framing jig that Mustang Steve built for another project. I am mostly there I just want to make sure before start welding. It just looks slightly off where the front meets up with the firewall. I need to attach the convertible top and make sure with it opens it meets up with the top of the windshield. 
 

On my 66 coupe the rockers inside measurement matched the above drawing.  You can also see in the drawing the front frame rail center line is 30" front and back of the front rails.   A simple centerline would be the best way to set the front measurements.