Can anyone give me any insight into what would be required to get a Viper Engine to produce 680bhp? Off the top of my head I can come up with racing exhaust, stronger crank/conrods, hotter cam, remap injection, not sure if bigger valves are available.

No need for stronger internals. Do some basic mods (exhaust, intake, etc) and a shot of NO2 and you are there. If you don't like NO2 just go with a bigger came, some heads, and the previous mentioned mods (excluding NO2). Not very hard to produce 600+hp on a Viper.

Thanks SL3000GT
As I'm in the UK and there are few if any Viper tuning people here can you recomend any suppliers of modified heads, valves, exhausts etc?