Re: thorley header
I'm not totally positive on this, but, I'm pretty sure the Thorley has an o2 sensor hole as it is 50 state legal.
2inch or 2 and a quarter inch exhaust should be good with the header. You can get a high flow cat from Summit racing or any local auto store that carries racing stuff (being in central north carolina we have a lot of race shops around), same goes for a high flow muffler, flex pipe can be had at any auot parts store. One note on the flex pipe, I've heard from some people that have negative effects with the flex pipe rotting out or tearing. Don't know for sure if that's a big prob as I have never used it personally.
88 Sammy in the garage