Oh yeah, used to have huge problems with numbers and spelling things out. These days I only have problems with "S" numbers (six, seven), but I know what you mean. Here's the thing with letters and numbers: you can't substitute anything for them. You have to just say the exact number or letter. If you're like me (I was once a consummate synonym user to avoid stuttering), this could make you feel backed into a corner. Then you begin building dread, which builds anxiety, which makes your stutter worse. It's a whole vicious cycle. I overcame it by not using synonyms anymore and no longer avoiding situations where I'd need to stutter. After so long, I became pretty desensitized to stuttering on something, which made it not as big of a deal to me, which lessened my anxiety, which actually gave me some more fluency. I still stutter badly on "six" and "seven", but I tell the person I'm speaking to that I'm about to have difficulty on these numbers, and that their patience is appreciated. Takes some of the pressure off.