The angle bracket used to attach the center rib to the firewall had never been countersunk, so I did that as well as dimpled the firewall.

Here's that figure eight hole.

I fabricated a little doubler. This is a view from the inside. The firewall is to the left, the upper forward fuselage skin is visible at the top of the photo, and the little doubler is clecoed in place just above the triangle cutout. I made it so that it would also pick up one of the pop rivets which will connect the rib to the firewall brackets.

After that, I put in some temporary, soft rivets to attach the side skins to the longerons. I'm going to need to lay the upper forward fuselage skin over this skin, and it just won't work if I have a bunch of clecoes at these locations.

With those firewall brackets riveted together and to the firewall, that pulled those pieces together a bit more than clecoes did, so I found that the forward ends of the ribs were slightly too long, which prevented the #30 pop rivet holes from completely lining up. I sanded and filed them all by maybe .020 or so.

Now there's a little clearance between the forward end of the rib and the firewall bracket. The pop rivet holes align nicely now.

Did some layout and drilling, and now the skin has been drilled to the center rib.

How to drill the skin to match the sub panel flanges? I could get upside down in the airplane, but realized I had this hole transfer tool available. I've had it a long time, but haven't used it until now. I better check it to confirm the alignment is good before using it on the upper forward fuselage skin.

Drilled a hole in one piece of scrap, so I could use the transfer tool to transfer it to the other piece of scrap.

Clamped the test parts in a vise. With the male nub through the hole in the under-laying part, I drilled through the bushing into the top part.

It worked well. The alignment wasn't perfect, but was within probably .002 to .003, so that should be plenty good.

Now ready to use that transfer tool to drill the first of many holes through the skin to align with the sub panel flanges.

After transferring four holes (one at each end of these center two flanges), I took the skin and flanges off and transferred the rest of the holes from the inside out.