Here are two pictures of a torque box. A torque box is essentially a box that joins the frame (unibody) to the car body, for strength I believe. Might be different for different years. I believe the 67 does not have a passenger side, but I could be wrong.
Looking at floor towards front of car. Torque box is next tom and in front of unibody frame rail.
I took the liberty of circling the area where I have a hole (see image). The part that looks like the frame rail is in great condition. The floor pans are fine also. Is the part circled replaceable?
Well the car was given to me so if this is the only really complicated part that is wrong, I am still ahead. Question.....What kind of a leak would make the bottom rust out of the box? The rest of it is fine as far as I can tell.
I think what 67.5xr7 is trying to say is (and I am taking some liberty here): you can fix it, by grinding out rust (inside and out) and welding a plate on it. This would give you the strength you need there. But, if you want a Concours car, originality, or resale value then you would need to replace the entire box. If the floor are in really good shape and that's the only hole, I would patch it (but that's just me).
As to where is came from, there are several likely sources. The boxes aren't really sealed, so water could have gotten in from the top and not drained out. Could have leaked down from cowl (clogged by leaves, etc.), or could just be where rust decided to take up residence.
Have you checked the floor under the carpet by the dimmer switch? Could be rusted there too. I have a hole that a bowling ball would fit in there, but I have a Wisconsin car
I have checked the floors because when i got it, it didnt have any carpet. Do they look like new? No but they are no where near bad condition. I am a structural designer for ships so I really understand structural integrity, it just makse sense. I like the fact that since I am not building a show car, I do not need to worry about total replacement. I will cut out the rust and repair as needed as long as it is structurally sound.
When i first gotthis car, i was like it kind of a neat car but the more I tinker with it, the more it becomes a labor of love. Thanks for the info guys, I REALLY appreciate it!
Usually the source of corrosion is road salts and debris (salty slush and mud)kicked up by the tires. The boxes are fairly well sealed as an installed unit. There are a few small holes,...fuel line passthrough, front lower corner near frame rail in fender well and seam gaps here and there. Got to keep the water out! http://www.mustangsunlimited.com/itemdy00.asp?T1=A114ALH+01
Not too much road salt here in beautiful coastal va. The fuel line pass through is one of those things tha ford did that doesn't make much sense to me. The timing chain cover/water pump arrangement is another. But all in all, I am loving it and thinking about what it will become rather than what it is.