Mike,
First off, Brownells Hi Force 44 is nothing more than 95% tin and 5% silver, you should be able to find something locally. You will need to find a good soldering flux that will work with it, though. That may be the part that Brownells won't send airmail. You can use rosen core electical solder for the ribs but I would not want to use it for attaching forend brackets or spacers.
Ellis Brown is telling you straight, he is a nice guy and was just trying to help you out. If you are serious about needing the help of this group, it might not hurt you to be just a little humble till you get the lay of the land. We are all willing to help out a first timer, but snide comments are best left out of our discussions.
I wish you well on your project, and if you need further help you can contact me by P.M. and I will try to answer any questions that you might have. As an after thought, I was told by a man who knows these things, that the British gunsmiths use straight tin to put on ribs and all the rest of it, fluxed on with powdered rosen put on by pinching of a little of it between their finger and thumb and sprinkling it on the hot metal. Bob H.