50/39 exists.
50/42 can be done by switching the 39T for a 42T from your triple.
check out
http://www.chainreactioncycles.com/ they have a sale going on for cranksets in the gearing you want.
WOW! THAT'S A LOW PRICE! (sorry, had to)
Just picked me up a 105 50/39 for $32!!!