1963 tbird camshaft eccentric, what is it?

It is what moves the lever up and down on the fuel pump to make it pump. If you're having trouble getting the fuel pump in, just rotate the crankshaft slightly by hand(make sure the key is off!) and it should go in. Here is a picture of what it looks like with the timing cover removed:

The eccentric is the silver item in the centre of the picture.

There is a lobe on the front of the camshaft that is called the eccentric that sticks up. Every time the camshaft makes a comlete rotation that lope is supposed to come in contact with the lever on the fuelpump thus pushing down on that lever and making the fuelpump work. The fuel pump lever must be inserted under that lobe or it will not work. The best way is to get the pump on correctly is toinsert the lever into the the opening on an angle pointing down, then push down on the pump towards the mounting holes (there will be some tension) and start one of the bolts and tighten that bolt about halfway. Then start the other bolt and tighten halfway, then go back to the first bolt and tighten down snug and follow with the other bolt alternating back and forth until they are both tight. Usually a little bit past snug should be good enough. You don't want to strip the threads or break a bolt off, don't forget the paper gasket first. Good Luck, Kenny / MrBlueOval

You wont do any better than the above answer. It is just a matter of lining up the pump lever.