Awesome, thanks a lot couldn't find this anywhere. So with the DT hole being 1.5cm in diameter, it is unlikely to fit at 1.44cm.

Well, thinking about it anyways. Don't want to order the di2 until I know if its feasible. Without having a discussion on frame drilling in general or whether I should just buy a new frame, etc., does anyone know the dimensions of the di2 internal b junction?

Looks like it would require three 5mm holes: One in the TT or DT, one in the ST for the FD, and one in the rear of the chainstay for the RD. The chainstay one already has a drain hole in back about 1.5mm wide, so I think enlarging that slightly would be the best bet.

In the bottom bracket area, the chainstay hole is large enough to fit the wire coming from the RD. The DT hole in the BB is about 1.5cm, and the ST hole in the BB is about 2-2.5cm.

Was planning on the standard routing with the internal B junction sitting in the DT, is the 1.5cm hole enough to get it in? If not, is there anything wrong with putting it in the ST assuming 2-2.5cm is enough? Looks like I can according to a GCN di2 install video I watched. Just have no idea how big the junction is. Though it makes sense that i the B has room for only two connectors side-by-side, and if each is 5mm, then it should perhaps fit into a 1.5cm hole...

It's a steel frame by the way with an English threaded BB.

You don't need to use a junction B, especially on a road bike.
Just use your ST- as the junction B.
1 Wire from the RD goes to one port of the Junction A 5 port box
1 wire from FD goes to one port of the Junction A 5 port box
1 wire from the battery goes to one port of the Junction A 5 port box
1 wire from the shifters to to one port of the Junction A 5 port box
1 wire from the shifter goes to the other shifter's 3 (2 remaining) ports.
This allows a tidy placement of the junction A externally at the FD or below the water bottle cage or even under the BB shell.

-SD