I'm a reasonably good bike mechanic. But so far, I've never had a freewheel apart to the level of cogs and spacers, though I've removed cogsets from bodies to clean and lubicate pawls and bearings.
I've been wanting a particular ultra-six freewheel setup for one of my bikes, but wide-range ultras are rare and very expensive. Corncobs, on the other hand, seem to be a lot easier to find. Would it be practical to buy a narrow-range ultra--say a 13-14-15-16-17-19--take the cogset apart and reassemble it with the cogs I want, taken from a couple of 5-speed Suntour freewheels? My understanding, quite possibly wrong, is that 5 and 6 speed New Winners, both ultra and regular, use the same cogs and bodies--that the only difference is the spacers. Also, that cogs from 5-speed Perfect or Pro-Compe freewheels can be installed on a New Winner body with ultra spacers.

Am I right or wrong about that? What kinds of frustrating problems am I letting myself in for?