That's correct about the 870 barrel and drilling the ports is a pretty easy job for any competent gunsmith.It's a lot easier to find an 870 barrel than a Model 58 barrel which in Canada are very few and extremely far between.
If you really need Model 58 parts though,email this fellow,the58sportsman@comcast.net.He's sort of the model 58 guru south of the border and he has a huge supply of Remington parts.I've dealt with him before and he WILL ship to Canada.
Dave