I've onlyy built two uppers in the past.  I'm building a rifle with my 16 year old son. I bought a railed gas bloch, and aset of Magpul hand guads for thehi build. My question is, will I have to shorten the gas block the thickness of the standard hand guard end cap in order for the gas holes to properly align?  Will I have to do anything special to keep the end cap from rotating?

I could swear I've heard you are supposed to leave a .032" gap between the back of the gas block and the shoulder of the barrel, even if you don't put a handguard cap on, and that this is for all gas blocks, but I can't prove it and I'm no expert, wait for someone else to chime in. I left a .028" gap on the last low profile gasblock I installed cause I couldn't remember for sure if it was .032 or .030. It works perfectly, leaving me to believe the gas port in the block is larger to allow some leeway. In other words, all gas blocks most likely take the thickness of the handguard cap into consideration and the gas port in the block is almost certainly bigger than the one in the barrel. I would think a railed gas block (or fixed FSB) in particular would take the handguard cap thickness into account as they're not designed to go under handguards.

The only way to know for sure is to measure the location of the gas port on the gas block and compare that to the distance on your barrel from the gas port to the shoulder. Some gas blocks make allowance for the hand guard retainer thickness and some don't.

Which block did you get?  Some are specifically designed to hold the handguard cap, while others are designed to NOT have a cap.  The manufacturer should include information about that in their instructions.  The gas tube will keep the cap from rotating.

Thanks for the information. Neext question do I need a different gas tube or is a standard 556 tube fine?

Properly align the gas block with the gas port in the barrel and use a standard gas tube.