The only 'common' Voidraven conversion I've seen is to use Forge-worlds (£66) Eldar Void dragon, which looks like this:
For venoms, it's typically a cross between a raider and a vyper, look around on P+M and you may find a tutorial.
Razorwing is usually either used via the same model as the Voidraven
AFAIK.
Many people use counts-as, for example, I know Blackmoor uses a Batman vehicle-thing to represent his Razorwing.
For what it's worth, there's supposedly a R-wing or Void Raven coming out late-summer...