There were two different sizes/types of radiators in the 67/68 Mustang.  Non A/C cars had a 20" wide unit.  It is realy easy to bolt in by drilling 4 new holes.

The best one to get is the 24" wide.  It is a clamp-in design.  The clamps will run you about $90 unless you can source some used ones.  That one requires some trimming of the core support, but is not a big deal. 

Here is a picture of the 24" in my 66.


I have most of the info you will need on my web site article on same.  http://www.mustangsteve.com/radiators.html

If you put one of those 24" units in your car with a factory shroud, it will cool sufficiently.  That is the same setup the 428CJ cars had.  I still do not understand why everyone wants to complicate the heck out of things with an electric fan, but to each their own...  As for aluminum radiators...I just purchased a new Griffin to replace the one in the picture.  (The body shop damaged the brand new brass one that was in it).  I looked around alot and Griffin was the only one that had tanks that would accept the factory clamps.  At least as far as I could tell.  Maybe there is a cheaper one that has them.  And Griffin makes two versions.  One has dual row of 1" tubes.  Other has dual row of 1.25" tubes.

67-69 radiators had lower inlet on passenger side.  70 has it on driver side.  Same hoses fit these radiators as the ones that fit the 17" radiators.