Mike,

     DeHaas (and others) show the low-wall thread to be .825" major diameter with a 16 tpi pitch, with this threaded shank portion extending .845" in length. If the barrel on the rifle now really is .790" X 18 tpi, it should almost fall out of the receiver I would guess.     

     Both large and small shank Winchesters used the 16 pitch while Marlin Ballards used the 18 pitch, IIRC, and for the Ballard there was an unthreaded portion to the shank while the Winchester folks threaded the whole thing.  Hope this will help and feel free to ask for more info as needed. 

Froggie