What have you been using as an assembly guide? There's an older book my dad had, How to build a small block Chevy, that I used for my first few.. Did you check the bearing clearances and turn it over before putting the cam in? What about before you tightened the rockers?

Flat tappet cams don't need anything as stated before, but if you had a roller cam it's necessary to put a cam button in, which is a spacer with a bearing that attaches to the timing cover or the front of the cam/gear.
Flat tappet cams typically require an iron distributor gear, while roller cams use bronze or other soft materials since the cams are a softer material.

Back to the rockers.. Turn it over and look at how they contact the tip of the valve stem.. Make sure it doesn't come off the tip or get onto the edge of it, as this will determine if your pushrods are the right length.

With all of this said, get a rebuild book. All of your questions will be answered in one simple reference guide for the low price of $19.99

-Dale