you already have a C6 as a very streetable "fast" car so having a viper thats very streetable and "fast" also is almost silly. you like to race and if you dont have a high HP car the C6 or the viper would eventually tun into one so dont waste your time and money.

TT big cube LSx C5 out of those choices.

in my honest opinion i think you should keep your engine plans but do it in a 98-02 SS/WS6. they are cheaper and are easier to make hook properly. yeah the C5 will handle better but if you want to road race or hit the mountains then the C6 would be better than a 1000+whp C5 anyways. yeah the C5 looks better but if you want to parking lot pimp then the C6 does that just fine as well. i think you want a stupid fast car that performs and in that case a TT big cube LSx F-body with proper suspension and good tires is hard to beat.