What headers will fit???

man, that SUCKS
you mentioned an installer, if this is a reputable company that installed it, they made a boo-boo and should be willing to fix it and eat the cost
if this "installer" was a buddy, well you might be out of luck

if I am not mistaking, the transmission mount can be spun around and installed backwards as well (though the lip interferes if the engine is in the right way, but I'm nut sure if it will if the engine is too far forward)

as for the driveshaft, I'm not touching that, but it could be could be that there is enough tolerance in there to run it, I would call a reputable driveline shop and ask them (of course, if a shop made this mistake have them make it right)

what are you running for a fan?
if your running a clutch style you are out of luck but if you are running a mechanical fan we may be able to run a shorter spacer and be good with where the radiator sits

toss up a few pictures of the fan set-up and well see where we stand