In a couple of afternoons I installed a 67-68 Mustang core support in my 65'. It requires some welding and body work skills as it is not a direct replacement, per se. The height and apron width are the same as constructed. There are some differences that require some metal removal and some that needs added in order to fill the gaps.
It will also require a battery tray mod which consists of a 67/68 battery tray and the support brace from a 65/66 if you want to keep the battery in the engine compartment. Another option is to use the 67/68 apron which mounts the battery horizontal in the engine compartment (instead of horizontal to the radiator support.)
Once the new support is installed, it would be easy to follow Frdnut's article to install and install a crossflow. Alternately, there have been people who cut out the raised portions of the existing 65/66 support to create a larger opening. This would certainly require a lot less work than what I did. This would require that you be a little more creative with the top mount.
Because I try to make my car a sleeper, I used a 4 row copper brass 67-68 AC radiator. I used a 67/68 top radiator mount and fabricated the lower mounts. Here is a couple of pics of my 65' with the 67-68 core and radiator installed:

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Dennis
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65' STANG (3330 lbs), 393W NA, Toploader 4 Spd, 4:11, AFR 205's, Vic Jr. Intake, CI Custom SFT, 750dp, 11.0 CR

11.25@122.19mph