Agree with what's being said about taking your signals from the 7442 rather than the 7406. Would an alternative solution be to use the analog inputs on the arduino instead of digital? Then you could measure the voltage taking TTL levels into account and produce your output from there. Might save you from having to deal with extra hardware.
I ran into a similar problem recently as the TTL spec defines Low as between 0 and 0.8V and High as between 2 and 5V, which is lopsided compared to what you might expect (Low less than 2.5, High greater than 2.5).
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