Hi Marc,
I didn't know of the Rover site, and it being called Birmabright, nor using the 5356 rods, but instead was using The Tinman's recommendations. His site is:
https://www.tinmantech.com/html/aluminum_welding_supplies.php of which I've purchased several videos. He does his repairs the old way, using gas. But using TIG is even better, of course. He recommends using ER-1100 and will sell it to you in a roll, 3/64 diameter. I used it in patching some cracks and body grind through, on my BJ8, as well as patching the holes in the rear shroud where the hinges mount. The aluminum is very dirty, as Andy mentions, and the darn stuff will oxidize in 20 seconds after you clean it...or so I remember from the videos. I didn't have any cracking issues when I did mine, but the stuff is so thin, you have to be very carful of over heating and dropping out a puddle of aluminum. I used the .040 tungston tips, and 3/8 or so cup and kept the heat down low (I have a Miller 165 diversion, not a high freq machine, but it seems to work). I didn't use any flux with the TIG, but I think that may help too. In talking with a welder at work, one who has experience with aluminum, he said to melt the metal on the edge, just enough to get the impurities to float to the top, then clean and repeat until no impurities are there then weld to that spot. I was able to fix the cracks and holes and so far, knock on aluminum, no cracking....