I planned to install modern air conditioning in my Montreal but then I sold the car, but kept the parts I had purchased. I found a parallel flow condenser just the size I needed (13"x 22", or 33cm x 56cm) at an auto junk yard here in the U.S. We have junk yards that allow you to wander around and find what you need. As it turned out it was also exactly the same size to use in my Alfetta GTV, and I went on and finished the job.
I do recall there is a center divider plate in front of the radiator on the Montreal that I would have removed, modified, or otherwise relocated so the condenser and fan(s) would fit behind it. I think the conversion would have worked out very satisfactorily especially since the Montreal "cabin" is pretty small. The Alfetta cabin and windows are larger but it cools very well.
All the other components (except the electric fans) were sourced from vintageair.com here in the U.S., and they have a lot of information and a complete catalog on their website. Yes, my system uses R134a coolant. I installed Vintage Air's smallest evaporator assembly behind the dash which meant throwing out all the Alfa parts but the V.A. system includes a new heater core with defrost as well as cooling. Yes, it was a lot of work, but worth the effort.
Vintage Air says the important things are to use the largest condenser that will fit, and to block off all outside air coming into the cabin.