obvious selections will be an open end-boxed end, but most of the time there's no access or if there's access, sometimes one person cannot hold the wrench and operate the impact at the same time (inside a frame member) if you place a box end inside a frame and jam it tight, as soon as you pull the trigger on that impact, the wrench will shake loose and fall into some deep dark hole.
for those situations that you cannot hold the opposing wrench and pull the trigger at the same time i prefer this hybrid kid: thats what you get when you leave your adjustables and locking pliers at the same drawer overnight:
Stanley 85-610 10-Inch Long MaxGrip Locking Adjustable Wrench
http://www.amazon.com/dp/B00009OYGZ/?tag=atomicindus08-20
i hate using ratchets against impact guns, impacting seems to rattle the selector spring out of position and put them perfectly into halfway, thus stripping the gears, breaking springs. but most often that flex head is the only way to reach. i try to put my fingers on that selector tab to make sure it stays put, but that rattling and buzzing feeling coming inside that ratchet makes your teeth grind.
to duplicate the access of that flex head, i prefer to use the 'swivel socket wrenches', instead of buying a complete set this works wonders for $10, you can add extensions between the handle and socket, more useful than that swivel socket wrenches:
Williams Flex Head SQ Driver 3/8" - 1/2
http://store.harryepstein.com/cp/WilliamsD/11920.html
on extreme situations where you have to go a full 'U' to hold opposing nut, instead of using the flex head and an extension, i try to use the a solid handle and two u-joints, u-joints lock nicely when flexed beyong their working angles, a useful feature in this case