The benefit of having a unit that completely replaced the existing heater box and defrost is you get chilled air through the defroster, which is a huge inprovement over original.

The benefit of a simple underdash hang-on unit while retaining the original heater is you get colder air.

I have Vintage Air. Ok stuff, one of which is 14 years old with no issues. Since I have not tried the other brands, I cannot tell you if it is the best one or not. I think it should make colder air than it does, but I think that is because it has heater and ac core in the same box.

Those who have the in-dash Old Air unit; how cold of air does it put out on a 100 degree day?  If mine makes 40, it is doing great.  Don’s under dash unit makes much colder air.

I have the Vintage Air Gen IV system installed in 2016 and am pleased with it.  You can download (PDF) the instructions and get a very good idea of the installation w/o having the unit in-hand. 

If you get Vintage Air, be sure to spec the system with a condenser designed to fit the wider 24” radiator. Even if you don’t use the wider radiator, it still works with the original radiator and is a much cleaner design as to how it mounts under the hood latch bracket. No holes needed for side mount brackets.
Plus, it fits with the wider radiator if needed.

Toploader wrote:

MS wrote:

If you get Vintage Air, be sure to spec the system with a condenser designed to fit the wider 24” radiator. Even if you don’t use the wider radiator, it still works with the original radiator and is a much cleaner design as to how it mounts under the hood latch bracket. No holes needed for side mount brackets.
Plus, it fits with the wider radiator if needed.

Thanks for the tip! Is the Vintage air the best system to go with? Is there a particular model to quote when ordering? Can I remove the heater box and just have the A/C unit provide the heat?

The install PDF will give you your best answer to the detail question:
https://www.vintageair.com/instructions_pdf/551964.pdf

 

I used Classic Auto Air on my Mustang and 1970 F-250.  I ordered both with a hose kit rather than the pre-made hoses.  The Mustang has a mid-mount compressor on the passenger side and it was easy to flip the condenser left to right and tuck most of the hoses under the battery or through the fender.  They were easy to work with and let me order exactly what I needed for the job.  The condenser is the standard unit that they provide for all early Mustangs.  I could have gone wider but it works and doesn't completely cover the radiator.  https://www.classicautoair.com/mustang_air_conditioning_ford_ac/