Now we are getting somewhere....
>so i will either use a bmx freewheel or a singlespeed freewheel >(are they just different terms for the same thing???).
BMX freewheel is just that it is a single cog on a FREE wheeling mechanism. Meaning that it is not fixed so if you want to make your bike fixed you need to get a "track cog". A track cog is a single cog similar to the one you used on your freehub project except it has threads on it as apposed to the cut out secitons for the splines on a freehub body.
You may be able to place a small spacer over some of the threads on the threaded hub before you screw your new "track cog" on the hub. Then you may want to try a bottom bracket lock ring which is the small ring on the (non drive) left hand side of the bottom bracket. It is what is used to lock the adjustable cup of your bottom bracket. Unless of course, you have a new fangled cartridge bottom bracket which doesn't use a lock ring, then you would be confused and not know what the heck we are talking about.
Once you have managed all of this you will need to correct your chain line which would be the most difficult part. You will likely need to use your chainring on the inside vice the outside and may need to remove some of the spacers from the right side of the axle and move them over to the left side of the axle. This would then make the rear wheel closer to the right side of the frame which would require you to redish the wheel.
>is it possible to space the singlespeed freewheel towards the >outside (like how i used spacers on the cassette body to space >my cog to the outside)?
Possibly, but just a little, the threads are quite small in comparison to the freehub body size. You only have about 3/4" of threads to work with so your cog will be way to the inside of the wheel once you get a track cog threaded on there which is why you would need to redish the wheel. This is where it would maybe be better to get a hub which is designed for this application. They make flip flop hub which allow freewheel single speed on one side and fixed track cog on the other using a proper left hand threaded lock ring which will prevent the cog from unscrewing itself. However I am sure you want to give this some testing before you plunk down the money to make a fixed gear wheel that you can't stand. I know I would.
John