I used to run dual display and it was a pain in the ass although I'm sure the technology is much better now. I'm not sure about an external HUD program. Look on sites dedicated to the game you're specifically talking about to see what people have come up with.

Though, if you make your resolution bigger in the game (provided your hardware can handle it) the HUD should get smaller while the FOV gets wider. At least this is how it works with most games.