reuse texture between quad

I want to spawn multiple blocks, each representing a letter. There will be more than one copy of an alphabet.

I have a big texture which contains all the letters in it. I use a prefab to Instance all the Blocks(quads) during Awake(). At runtime, I'd like to assign a texture to these quads (identifying the letter). My code looks like this:

     void Update () {
             // if there is no active block, spawn it
             if (activeBlock == null) {
                 this.getNextBlock (ref this.activeBlock);
     }
     protected Block getNextBlock(ref Block block){        
             block = this.deQueueBlock ();
             char letter = LetterMgr.Instance.getNextLetter();
             // based on the letter, index into texture 
             // and assign the texture + indices to this block ?
     }

How can I reuse this one big texture and index into each letter amidst all these blocks (quads) ? I will be rendering this on a mobile, so will need to squeeze all the performance I can. Details:

• 256 x 256 per letter

• 8 columns x 4 rows

• Total Texture Size: 2048 x 2048

• Total number of blocks: ~100

PS: Each of the letters will be moving and will eventually come to rest. So I will have some (around 6-7) actively moving blocks and others will be stationary.