Hi Ted. I am back from my holiday and read your confusion about the beam locations. In addition to what Daniel and Johan have already written, first start checking in 3 steps:
1: the position of the lower deck beams on the lower bearing beams 135. That is your base alignment! Like what is written in the (translated) manual pag. 11:
View attachment 450718
I high-lighted the numbers of the lower bearing beams over the frames 13 till 43.
Then you can see on the plan 8-1, fig. 9 what the position is of the lower deck beams 75-76, 77-90 and 91-101. They are placed
in front of the frames. Beam 75 is in front of frame 13, 76 in front of 14, etc etc.
For an extra check: beam 91 is in front of the frame 31 with the 2 holes for the pumps.
If that is correct:
2: In the bow, the middle deck beams 66 till 74 are positioned
in front of the frames 4 till 12 and are on the middle bearing beams 136. See plan 8-1, fig 11 and pag. 14-1 of the translated manual.
View attachment 450755
In this picture I also highlighted the position of the 4 blocks between the middle deck beams.
If that is also correct:
3: The upper deck beams are placed on the upper bearing beams
behind the frames: 126-1 is behind frames 1, 126-2 is behind frame 2, etc etc.
This drawing is from pag 15 of the manual where I named frame 1 and beam 126-1:
View attachment 450759
If those beams on the 3 decks are all in the correct position, then both mast-steps and the openings for the masts between de deck beams would be in line.
Hopefully this will help you further.
Regards, Peter