How to verify if an A-body is numbers matching?

Well, since this is a convertible you are talking about, areas like the floor area under the rear seat and in the trunk where the top folds into the well should be checked over very carefully. The rain trough for the back window and top doesn't really do that good of a job keeping water from falling into that area in the well and usually results in rusted brackets and metal under the rear seat. The 67-69 Cudas also have a penchant for rusting on the top of the inner fenders and the flange along the top of the fender where it bolts to the inner, under the hood.

Yes, there are numbers on the drivers side pan rail of the trans that will give you the part number and 10,000 day calendar to pinpoint your tranmission's origin. Also, there are date codes on every component if you really have the time to run numbers and find out what's been replaced along the way, but unless it's a rare car it's not really that necessary to have a perfectly correct car. If you can come close with a 340 car or a Formula "S" it helps to have correct stuff, but just a regular production 318 or /6 car, not so much!! I'm sure you'll be changing things to suit your taste anyway, so as the great philosopher and thinker of thoughts "The Burger King" once said..."Have it your way"!!!

The undercoating on the car will have been applied after the paint so it should be a grey/blackish color and will probably be very hastily applied if it is factory, which it will probably be. I think you're going to find that when inspecting a highly documented car that the car itself will either back up the documentation or dispute it. Things like a new paint job, overspray, areas in the trunk and floor pans that show that the body has been worked on will all show themselves unless flawlessly done, in which case you needn't worry about them anyway. Are the pedal pads worn or do they show wear indicative of the mileage of the car. Has anything been changed like the intake, wheels, exhaust?? Are there signs of new gaskets in the motor that would indicate that it has been opened up??

Documentation can also be meant to include receipts for new parts or procedures done to the car through the years. It doesn't necessarily mean that the car is rare or any more valuable, but it can lead you to an interesting timeline of events that will help you identify the status of the car. Things like oil change receipts not only show that the car was maintained, but having the dates and mileage on receipts is a good way to track the cars use and how much time passed between maintenance, showing that the car was driven sparingly or an everyday driver, or maybe how an event such as a motor problem sidelined the car or changed its use.

Above all is your comfort level with the car. Make sure it's what you want and is at a level in which you can enjoy it. Don't get something so rare or low mileage that you can't enjoy driving it if thats what you want to do, and don't get a car with a lot of maintenance issues if you live in an apartment...well, you know!! Good luck with your purchase, any convertible is worth having IMO!!! Geof