You could try searching at mustangandfords.com.

It's pretty easy.  You drain the radiator, cut the hoses at the
firewall (because you're goint to replace them anyway), detach the
control cables (label them and take digital photos), unplug the wires
and unscrew the tinnermans nuts that hold the motor to the firewall,
and the whole heater box is now free to drop into the passenger
footwell.  Now pry all the clips off the box and it separates like a
clam shell, and you are now in a position to replace the heater core.

180 Out

Here is a link you can try. Do a google search and you may find more help,
but it is pretty basic.

http://www.sacramento-mustang.com/Newsletter/Heater_Core/heater_core.html

I replaced my core recently wthout problems. I spent the extra time to
replace all the foam seals and replaced the cardboard plenum with a plastic
one while i was at it.

If you have a leak right now, you can eliminate it quickly by doing the
'mustang loop', which means simply bypassing the core with a loop of hose
from the outlet to the inlet. This gives you more time to fix the heater
without having to deal with dripping coolant.

Good luck.

Michel.
65 convertible 4bbl dual exhaust
(red, naturally)

Word of caution, be sure to drain your coolant below the level of the heater
hose outlet and inlet before you start, or you will end up with coolant all
over the place! Even so, remember that there will still be coolant in the
core and hoses, so be prepared.

Michel.