I think it should fit...
Miss Susie, I have a Ruger (Lite) 22/45 with a 4.5 inch barrel. The outside of the 'barrel' is actually a shroud, and non tapered. Attached is a photo of the pistol in question for your edification. (I don't know if yours is the same or close to being similar.
I do not possess a Government tanker holster. However, I do have a regular G.I. belt holster. My Ruger with 4.5 inch barrel fits the G.I. belt holster. (I checked prior to posting this.) The front sight does rub heavily. Keep in mind the G.I. 1911 or A1 have five inch barrels and a slide somewhat less in circumference.
You mentioned soaking the holster in water to 'fit' the holster to the pistol. Yes. That does work. I find using rubbing alcohol does the same thing to the leather without the risk of rusting the pistol. However, it does have a drying effect on the leather, so after fitting and such, one should apply some sort of lanolin type preservative to the holster to counteract the drying effect of the alcohol.
Oh. When fitting a holster in this manner, don't let the holster completely dry with the pistol inserted. Sometimes the pistol can be stuck rather tightly under those conditions.