Where can I find a replacement door / trunk key for 1974 Pontiac Le Mans?
Making a key from the glove box will get you just that: a key to the glove box. That lock only has 4 of the 6 tumblers necessary for a complete key. The locksmith must them do a progression to figure out the other two tumblers and must have the car or one of the other locks to do it. The procedure is actually the faster (and cheaper) way to make a GM secondary key vs pulling/disassembling a door or trunk lock. Key codes are not available by VIN that far back.
Why in le mans racing do they race 2 different class of vehicles simultaneously?
First off, there are currently more than two classes, and none of the cars are street legal - all cars are dedicated, designed, and built race cars.
The reason they run multiple classes can probably be summed up as "that's just how it's done in sports car racing." Though aside from that's how it's been done historically, most of the reasons that led to it happening originally are still applicable today.
First, as has been mentioned, is that endurance racing is a long event. Even 3 or 4 hour events would be two long to stage four or more seperate races. And even if there were time, how to promote and sell tickets and sponsorships for each of those races in order to cover the costs of carrying out the races. For races that take place on closed off public roads (as Le Mans), this would also mean a few extra days of road closures (if not more).
Second, there is filling out the field, and closely connected to that would be the third, keeping manufacturers interests in mind. Be it privateer or factory backed efforts, if you look at some classes, you'll find them to have very few entries... sometimes a classes that are going away or soon to be done away with for various reasons, you'll find only one team entered (imagine the excitement of watching a 24 hour race between two teammates and no one else even on track). Most often though, there is a few competitive cars in each class - enough to keep a class interesting, but not enough to provide a full field to fill a race track (let alone a long track such as La Sarthe.
Which leads into that third point... the shape and nature of sports car racing is often defined by the manufacturers involved - either through their factory efforts or by the cars they are selling to private teams. In either case, they have a major stake in the large races, and success in such a race (of which Le Mans is definitely one of the biggest) translates directly into money. Early on, if an overall win was the only way to achieve success, then companies that did not make cars with 5 or 6 litre engines would have had no reason to invest money in developing cars to race there. But in having multiple classes, there were still ways to be successful against something similar. This remains a major issue today.
What is the difference between Le Mans and Grand Prix racing?
An F1 race is run over a maximum period of 2 hours. Le Mans was originally a race in France held over a 24 hour period.
Effectively F1 is a sprint, whereas Le Mans is an endurance event.
EDIT: nice one Jenk, very thorough.
@Jeff Gordon's Gay "LeMons"? Maybe you should learn to spell or you'll look like a right Mons Pubis!! :)