Like a phoenix rising from the ashes, against all odds and whodathunkit to boot, you can now get 6 and 7 speed freewheels that are better than 99 percent of the freewheels from the past.  IRD is the brand, and this just opens up worlds of possibilities.  For instance, now you might be able to afford a Phil rear hub, since a freewheel-style one of those is about 1/3 the price of Phil Cassette.  Think about that, man.  We'd recommend the 7-speeders, unless you have a particular reason for 6.  But you know, you could get a 6 and get a Phil 135mm rear hub and build up a near-dishless wheel on a Mavic rim (Mavic doesn't make asym rims, so that's the only way to do it).

Use Park tool FR-1 (Shimano style) to remove IRD freewheels.

Part No.      Teeth Count
13-091 :  13-15-18-21-24-28
13-093 :  14-17-20-24-28-32
13-092 :  13-15-17-19-21-24-28
13-094 :  13-15-18-21-24-28-32

If you're planning on increasing the amount of speeds from 5sp or 6sp to 7sp (or whatever) on your bike then be prepared to at least reset the limits of your deraileur and probably switch around spacers on your axle and redish your wheel if not also needing to get a longer axle and increase the width of your rear triangle (steel frames only).  It may turn out to be a pretty involved and time consuming project to just add one or two extra gears.

Here are some additional measurements that may help you determine if these freewheels will work on you plus one (or two) project:

Thickness (measured from the outside of the high cog to the outside of the lip of the thread body, beyond the low cog) of the 6 speed freewheel is 31mm; the 7 speed is 33mm. 

Thread body is 12mm deep for both 6 and 7sp.

But the best way to determine whether or not a new freewheel will work on your wheel and in your frame is by trial and error.  Good luck.