I did my first conversion this weekend and simply swapped the freewheel on my 1982 peugot for a cog and BB lockring. After screwing around for hours with the chainline/tension (all big changes at the crank end), I managed to get it rideable. It is nothing to brag about but I am pretty sure it is safe and have enjoyed the first 30 miles on it without incident. I did almost go over the bars a couple of times on a 20 mile group ride when I forgot about the bikes new rules. Also, going from advice on this forum, I rode it hard and then re-tightened the lockring.
I notice, however, that I see almost no conversions that have simply left the rear wheel as-is. I know it would be stronger to re-dish, but I am not certain I will never want to replace the freewheel. Moreover, I think I would prefer to save up and buy a nice set of wheels with flip-flop hubs or something and a real reverse-thread lockring option.
Is it safe/sound to ride without re-dishing? How precise does that chainline really have to be?