The c4 is a much stronger trans and the 2.0 c4 bell can be bolted to a 2.3 with the use of step dowel pins like these http://www.turbopinto.com/index.php?topic=411.0.

Only other person i know that sells them are Paul LeDuc Over on Turboford http://forum.turboford.org/ubb/ultimatebb.php?ubb=get_topic;f=6;t=014626 But at the moment hes in the hospital he fell down a flight of stairs during the hurricane so he might not be able to sell them for a while.
 
Id bet you can drill the holes in the bellhousing just enough to make the 2.3 dowels fit or if you know of any machine shops have them make a couple dowels all they do is go from the larger 2.3 dowel and reduce to the smaller size of the 2.0 bell dowel hole.

So dowels are the only challenge? Everything else just bolts up?

You may also need a flex plate depending on the vintage of your C3.  My 72 Pinto 2.0 C4 used a 4 stud torque converter while the 79 Pinto 2.3 C3 used a 3 bolt torque converter.  I have seen a 90's Mustang 2.3 overdrive C3 with a 4 bolt torque converter.  I believe I have read that the parts store replacement 2.3 flex plates are drilled for both patterns, but check before buying.