I tried some of those low temp aluminum brazing / soldering materials without success. Had trouble fixturing the muffler during assembly as aluminum conducts heat so well it was easy to disassemble previous welds doing the next weld. Once assembled, welds quickly failed. When I work with aluminum these days, I just have it TIG welded by a pro and the cost is still less than a commercial product. Maybe you will have better luck than I did.

Steel is much easier to work with and is easily welded with an oxy / acetyl torch using steel filler rod. When I can find it, I'll post a photo of one muffler I made for a scale Zero.