I do not own an SBR, but I have lurked and posted for years on http://www.subguns.com
and read a bit of the CGA/NFA myself. Here is how I understand it.
The lower receiver is the part that is engraved and registered with the ATF. The upper receiver and barrel do not require anything. The serial number is required to be at least 1/8th inch high and .003" deep, so this is safe for the rest of the engraving also. You can get the ATF form 1 from the subguns or ATF websites. Make sure you print out the forms double sided and fill in all of the blocks that are not reserved for the ATF, even if it is just "N/A".
You might get better answers from the Subguns NFA forum, but it is an old style format and the answers go away after a matter of days and become harder to find in the archives. Good luck.