Will this Q-Jet carb work on a stock (with headers and HEI)'73 400/TH350...Its running the 400 but is in need of a rebuild, will a kit to suit a 400 fix it or do i stick with the 350 Olds kit, any advice either way would help and save me spending cash on something i dont need

Best thing you could possibly do is buy this book first:
http://www.amazon.com/Rebuild-Modify-Rochester-Quadrajet-Carburetors/dp/1932494189

And all those questions are answered in that book. But the quick answer is YES, it can be made to work fine. But don't buy your kit from any old place...after reading the book contact the author and buy your parts from him, he'll sell you exactly what you need for your exact engine:
http://www.cliffshighperformance.com/

I just did his comprehensive treatment on a 1977 GMC smog-era Quadra-jet for my Pontiac 400, and I am 100% pleased with the results...

Thanks for that info and the link...email sent...