I did end up trimming all the boards. They are all now 100 mm wide, with the short letter boards being 98 mm high, and the tall letter boards being 130 mm high.
I ruled lines across the boards for the letters to ‘sit’ on (to help orientation, and also to model correct positioning). On the short letters and the tall letters that extend upwards, this line is 25 mm from the bottom. On the tall letters that extend downwards it is 58 mm from the bottom.
I have positioned the letters on the line and central. I decided to do this so that they will suit both right and left handed users.
I did the gluing in batches of about eight to ten cards. I would lay each card out, select the letter and check its position (so that I considered its slope for example). Then I turned the letter over and used PVA glue to dot along the back. I spread it carefully over the whole back of the letter, making sure that it wasn’t too thick. I then placed the letter on the card as accurately as possible and patted it down with a tissue to collect any excess glue.
I finished by turning the letter over onto the table, and weighting it down with a large book to ensure that the card and letter didn’t curl away from each other. After they were allowed some time to dry (about 15 minutes I think, the glue wasn’t very thick) I collected them up and allowed them to stand between books on the shelf.
Now to make/find a suitable box to put them in:-)