Dave;
If your car has had no front end damage to cause the tubes to bend then what you are looking for is rust. Even though the car has only 42,000 miles it has 45 years of exposure to the elements.
My car had twice the mileage, no accident history and had been in the family since 1973. I didn't think the frames would give me any problems and as I took them off I noticed some sound of rust as I turned them from side to side and top to bottom. I washed them with soap and water and used compressed air to blow the inside of the tubes dry. I saw bubbles coming from the areas of joint brazing and on closer inspection found 'pin holes' in several of the brazed joints.
From what I have read that makes these frame rails junk. Remember these rails support the engine and front suspension and therefore are important for my safety and everyone I meet with the car. I ordered new frame rails from E Type Fabs in the UK.
I would suggest you spend some effort looking at the frame rails with an eye to proving them bad. If you can't prove them bad then you might consider them good but you have to work at it.
Good luck, Joel.