I can’t help in the realm of total costs, but it is certainly possible to have a well functioning A/C system on an older car. I recently completed a retrofit to R134a in our 1966 Ford LTD with factory air con. There’s attention to detail that needs to be observed when doing a retrofit. I’ll touch on some of it, but I don’t wish to bore any non-technical folks. As far as sizing of OEM air con components, the 3rd generation Ford full size models are adequate and I would wager a large sum of money that your 4th gen is also more than adequate. This includes the compressor, evaporator and condenser.
Here’s a simple run down of what I did:
I replaced the original York 210 with a brand new one. They still make the old York 210 brand new and for about 160 bucks for one is a much better deal than labour and parts to rebuild an old unit. Also replaced the clutch bearing.
The receiver drier was replaced with a brand new one made for the car and the desiccant is rated for R134a and its oils.
All hoses and fittings were replaced along with the expansion valve with one from a Mustang (long reason why).
The evaporator and condenser were thoroughly flushed with lacquer thinner to remove all mineral oil and clean the interior.
I recharged with R134a and ester oil.
The end result was rather good. On a 90+ ˚F sunny day down here in New Mexico, at idle, with all the windows open and the blower on high the vent closest to the evaporator (centre vent in dash) was blowing 37˚F degrees. If it ran any colder the evaporator would risk icing up.
Now I have heard horror stories about retrofits. Super heat is extremely important to get right, problem is some expansion valves are not adjustable so you have to retrofit one that is or one that is set differently. I can’t stress the importance as well in having a thermostatic fan clutch and factory fan with shroud spaced properly and working properly. Our LTD is a factory 390 FE with the original fan, clutch and shroud.
In doing the work myself, the part costs to me was around 600 dollars. So add labour and mark up on all parts and I’d estimate a grand to make sure it’s done right. Trying to adjust the superheat might take longer if you have to and hence more money that’s even if you can find a shop whose willing to do that. I was lucky in that the superheat on the Mustang expansion valve I used was close enough to yield excellent results.
Hope that helps a little at least.