Been reading about this swap, obviously. 
I'm  having trouble figuring out how to phrase my question.
I'm trying to look at this issue from another point of view. 
And again this is for a 74 (and up I suppose). A lot of what I have seen talked about here is for the earlier years.

Is the major issue with fitting an exhaust for this swap the motor mounts? The crossmember? The ground clearance? Clutch? Firewall too close? Other?

I am likely going to be using mounts that attach to the heads, so I am eliminating the frame mounts. My thinking is that when I get to this point I would like to get a set of used headers, maybe from several cars and see what happens. But if the problem isn't the frame/engine mounts then I don't want to waste time and money.

I have seen two options that might work, one being the shorty headers and swapping them, but then you have the exhaust running under the crossmember. Bad for clearance  in areas that have lots of speed bumps.
The other being using an early (I think it was 65) Mustang stock exhaust manifold that goes straight back instead of down. Then you just have to have the pipe custom made to meet up with it, which isn't a big deal. But then that isn't a header.  but it would let me use at least one vintage Mustang part, which is never a bad thing.

thanks,
Russ

 

Gee, only $370ish with shipping. 
I'll put those on my Christmas list. 
Russ

Not long ago I looked at a picture of one done with shorty headers, turned backwards.
Looked OK & the single exhaust (from each side/collector) goes under the cross member.

Lots of ways to do it.

From what I've seen the biggest problem is room, an engine bay that was not made for an engine that size has room issues. Even the parts made for a v8 swap have some room issues, some header tubes inside the frame and some out side,some go forward some go back. All I can say is just keep looking at all the ways people have done theirs till you see one you like.I had mine custom made for the fender wells. Bud