jmc

We could view the sum as the sum of a bunch of 6's, but a closer look simplifies the sum considerably. Both $-745$ and $+745$ appear in the expression, as do both $-742$ and $+742$, and both $-739$ and $+739$, and so on down to $-496$ and $+496$. These all cancel out, leaving $751+748-493-490=\boxed{516}$.