If the express reefers ran in passenger trains, they had to have "better" trucks. At the minimum bigger bearings and probably better suspension setups.
If they were solely in freight service, maybe later in their lives, then freight trucks would work.
My opinion is that they are more fun to model as part of passenger trains and hence require the express trucks.
You might be able to use Atlas express trucks if they sell them separatley. They are probably in the 20 something a pair. Another source would be Precision scale. They do a number of trucks in both plastic and brass. The brass ones are probably 40ish and the plastic 20ish and these are both kits.
Finally you will eventually find some at O Scale train shows, it might take visiting 3-4 shows but parts often show up on individuals tables. You could use old Walthers versions or any of the old brands. The bolsters would be at the right height from the rail.