The front drum applicable for my car have part number 667102 at VP. I didn't think it would be different, but I see now model year 1959 and onward have part number 667126.
Had a browse through the Swedish Amazon forum and came across this 1963 bulletin:
https://volvo1800pictures.com/vm_1/c...kivbromsar.htm
It explains that sport models already are fitted with disc brakes, and drum brake cars can be retrofitted with discs. Looking at the parts, it is as I thought. The TL;DR is basically to remove the old brake parts, fit the new, bleed the brakes, done.
There's also some mention in a thread that the early disc brake cars came fitted with too large rear wheel brake cylinders. This resulted in the rear brakes locking up when braking hard. If retrofitting disc brakes on an older car, these came equipped with the proper wheel brake cylinders in the rear. If not, the recommended size is 7/8".
Looking through some threads, there's no indication that retrofitting disc brakes require any suspension changes.