I have the same issue; 80s Ross 12-speed. I want to convert it to single speed before winter, for all the obvious reasons.
Here are the options I came up with:
-strip off derailleurs, keep cassette/chainrings on. Well, you've already done that, so...
-convert 6-speed freewheel to single speed, and take off outer chainring. Although you hear people recommend this quite a bit online, in reality it's not a great option at all. Every bike shop I've gone to said they won't do the procedure, because it simply isn't safe or reliable (when you change a 6-speed freehweel to single speed, you have to "re-dish" the wheel, which with that drastic of a change isn't good)
-buy a new rear wheel, keep front. There are two problems here. If you buy a 27" rear singlspeed wheel, well, they don't exist. The only place I've seen them is on the same site mentioned earlier, however I wonder if they have any in stock. YOu could also replace rear 27 with 700c, but then your tire sizes will be different and your bicycle will handle differently.
-get a new wheelset, 700c, singlespeed. This is obviously the better option here, and you have quite a choice with types of rearwheels (single-sided SS, free/free, free/fixed, fixed/fixed, single-sided fixed). You can buy the wheels, or make them yourself. I've seen wheelsets as low as 100, which is fairly cheap (questionable reliability)?
Let me know what you come up with, cause like I said, I'm in the same boat!