still very tempted to deliberately misspell fourty

Don't. In fact, I highly recommend making it just the numeric version "42".

Since you're going for the humor effect, you need people to react to it in the proper way. It's hard to do comedic timing in text, but in this case, your choice of how to print the word give you some control over that. The reader's brain needs to interpret the result as quickly as possible, and when seeing just the word all by itself, the number is going to hit their brain stem quicker, and make the Douglas Adams association quicker, than the full words would.

It would be different if it were a long piece of written prose. But since it's a tiny triangle in the 8-ball window, I think the number would work better than the words.