Here are some shots of my 65 coupe engine compartment
showing the bigger radiator installation.  I used a 1969 4 row -
high efficiency core radiator and the 69 fan shroud with a clutch
fan and an 8 blade flex fan.  It never gets hot, even in 100
degree weather, in traffic, with the a/c on!.  It pretty much bolted
right in...with a few mods.
First, I cut 1-1/2" from each side of the hole in the core support.  
(You don't HAVE to CUT - it will fit but the outer two rows of core
will be blocked)   Then spaced the radiator up off the frame with
1/4" wood spacer (will be removed after bolted in)  Then drilled
new holes for the 4 bolts holding the radiator in the car.  They
worked out to be exactly even with the existing holes that were
removed when the core support was trimmed.  I "percussion
engineered" the core support a bit to move the mounting flange
forward about 1/4" to allow for the radiator's extra thickness.  The
fan shroud bolts directly to the radiator, after drilling 4 holes.  If
you use a 3 row radiator, the holes are already there.  Then the
clutch fan and new clutch were bolted on.  Now I have a 4 row,
20" wide radiator in place of the old 17" wide 3 row job.  And I now
have a fan shroud that actually works.  My fan is a 17-1/2" 8
blade type, but I think an 18" would work even better.
I had to make some new brackets for the A/C condenser as it had to be pushed forward
about 1/2" and the original mounting holes were no longer in the core support.  I also used
a 70 model battery tray and fender apron to reposition the battery 90 degrees.  This is not
absolutely necessary if you trim the hold down clamp flange off the original 65-66 battery
tray.  Mine does have more room, thoug
h.
20" , 4-ROW CORE RADIATOR in a
65 Mustang
These can be found in:
67-69 Mustangs  - lower hose on passenger side
70 Mustangs - lower hose on driver side
Shrouds are available new  through MustangSteve for $90+ shipping
24"  CLAMP-IN STYLE
3-ROW CORE RADIATOR
in a 65 Mustang
Battery tray is modified using the brace from under the 65-66 tray and
the top shelf part from a 67-70 tray.  This will allow the wider radiator
to fit and also allows use of the superior 67-70 battery clamping
system.  You can use the long studs on both sides of the battery or do
as I did and use a bolt into the radiator core support for the front side.  
That is a simple flat peice of steel across the top.  I used some thin
rubber under the steel so it could not slide.
The Proxima battery with rounded corners is much easier to install.  
Square batteries require the radiator top clamp to be loosened and
the radiator slid to the side a bit in order to get the battery in the tray.  
Hoses need not be disconnected.
Radiator fits like it was meant to be there.
Upper clamp is from 67-69 Mustang or
Cougar

Lower clamps can be fabricated or modify
some from 67-70 Mustang with 24"
radiator.  Make them so the bottom of the
radiator just barely clears the frame.

Rubber clamp inserts are available
through Mustang parts vendors, or you
can just use some strips of heater hose or
insulation.

Fan shroud bolts to sides of radiator.
Small dent in corner of fender apron
lets the battery tray fit tighter.
If your car has air conditioning, the outlet tube won't clear the wider radiator.  You
will need a condenser that has the driver side fittings pointing straight down.  
Good news is there are some aftermarket units available for around $100 that
are larger than the stock original and will fit perfect.  Use an inline drier and route
the hoses down below the front valance to hide the drier.  New hoses,custom
made for your car are less expensive than you would think.  I had all my hoses
made from scratch for under $100.

Voltage regulator must be moved about 2" towards the fender
These can be found in:
67-69 Mustangs  - lower hose on passenger side
70 Mustangs - lower hose on driver side
Fan shrouds available through Mustang vendors
Click on this picture for a larger image
that will show you where to cut the core
support if installing the 24" radiator.
click picture for more details