Text Entry in Cryptogram Puzzle - help please

I am building out a cryptogram puzzle for a module and I have attached the file to help with this. I feel like I am close to my solution but can't quite figure my way out. 

At the top, I have an alpha key that is assigned numerical values that are completely random based on the website that I used to create the puzzle. I am trying to make this all fillable for e-learning. 

For the phrase to 'solve', I was going to put text boxes with references to convert that numerical value and place the alpha letter in the puzzle to help the learner. I have it working a little bit so you can see what I am going for. I put lines on the text boxes so I can see all the boxes on the screen for testing purposes only.

I would like the text entry field at the top to keep the numerical value assigned in the box and only change the text boxes in the puzzle to an alpha letter to solve the phrase. I would also like to not accept incorrect values. So letter A = 5 in this puzzle. If I put in another number, it puts that number in the phrase instead and I don't want that to happen. That would confuse the learner.