If you need a visual index to help time the barrel and the front sight, the threads on a steel USGI segmented cleaning rod match the front sight threads. Join a couple cleaning rod segments together and thread them into the threaded hole for the front sight. Now your front sight post is 2 feet tall.
If you want to create a perfect vertical reference, tie some (white) kite string to a socket or wrench and hang it from the ceiling or a rafter in your shop.
Aim the barreled upper at the white string.
Then you can level the receiver itself horizontally with a torpedo level.
By shifting slightly left and right, make sure the kite string aligns with the cleaning rod section at at both the top and bottom at the same time.
Clamp the front sight in place, make an index mark with a pencil so you can see if the front sight shifts while drilling, and you are good to go.
After drilling the first hole, I put a same-size drill bit (shank first) into the hole, that way nothing can shift while drilling the 2nd hole.