The median income per worker in the US is around $35,000.
We'd each be earning $42k a year more, not just a straight $42k.
The absolute middle-of-the-road wage earner would be earning significantly more than DOUBLE what they are earning now.
And IIRC, this $42k number comes from the RAND corporation a couple years ago. These folks are no analytical slouches, nor are they known as a liberal organization.
How are these numbers calculated? As I understand the median income, it can be lowered simply by adding more people. If the size of the economy remains the same (or merely fails to grow at the same rate as the population) and you split it among more people, each of those will earn less.
There are now 130 million more people in the United States than there were in 1975. This doesn't translate exactly into that many more working age people, but does give you a rough idea how much larger it has become over that time period.