The question when to switch from one to two masts, is not that easy. The answer depends both on the boat design, the crew, and the intended style of sailing.
The Cheoy Lee 38 should take a quite tall sloop rig better than most. She is big, with almost eight ton displacement and with 40% of that as ballast. Plenty of beam and draft should ensure power to carry a healthy rig. Even more, there is a good mast position waiting for a sloop JR mast - through the foredeck hatch and between the berths. This position calls for fairly short battens and thus a moderately tall rig. Such a rig is the easiest to deal with, area for area, as long as the boat has sufficient stability. As can be seen, the foredeck will have plenty of free room for anchor handling.
Now I had a go to see how it looks. The short chord of the sail is only 63% of the waterline, which indicates that downwind steering should be easy enough. In addition, the moderate length of the battens reduce the load on these quite a bit. At 5.95m length, they will make good use of 6m or 20’ tubes.
As for the workload on the crew, the hardest bit is no doubt hoisting the sail. However, my friend Svein Magnus in his Samson has hoisted his 70sqm mainsail by hand for the last 20 years, so it is doable. Even so, I recommend fitting an electric capstan of some sort. They are much cheaper than the extra cost of fitting a second mast and sail. I recommend dividing the sheet in two, with an upper and lower section.
If one is about to round 70, a big sloop JR may not be a good idea, but then it may not be a good idea to take on an 8 ton boat either...
The ultimate stability of that vessel should be around 6100kpm (Disp. x beam x 0.21). The mast should thus be built to take a bending moment of between 12200 and 18300kpm.
Something like that...
Arne