A 450WHP autocross car will blow up most 5 speeds, even with aftermarket gearsets. Motorsports dog box maybe, but that will probably cost $6k+ to do right, which is around the same price you'll pay for a 6 speed plus buying a synchro gearset you want.
You say this is an autocross car, but you have a 35r with no AVCS? I guess it's not a very competitive autocross car then, so just what is it you are trying to get out of your transmission? Really tall gearing for MPGs? A specific gear for autocross? Seems like for that kind of turbo with no AVCS, a 7k redline is severely limiting you (which is why Fuji is asking questions on that, you can fix some of your gearing issues by being able to run an 8k redline.)