I'd recommend getting PipeMax in regards to exhaust specs (most notably the header design and total length). Megaphone is a bit more of a black art, but you can use this as a guide:
http://victorylibrary.com/brit/mega-c.htm (about half way down... it goes over the 'Blair' method).
I think a 750cc XS motor would really respond well to a 1.625"od header stepping up to a 1.75" od primary before the megaphone. I know the SR motors really liked it (vs. a 1.75" od header), and they displace more per cylinder and rev just as high. The spigot is great since it removes the ledge at the port. Mine tapers out from the port diameter to the id of a .040" thick 1.625"od pipe, over about 1.5".
Take a look at Richard Pollock's XS650s trackers, and you'll see a stepped header.
The cool thing about a real meg set-up is standing behind it. It feels like an air canon which you can feel from about 8' from behind the bike. It also sounds really deep and boomy.