Hey mate,
Use a salisbury 4HA axle from a reliant scimitar. I know you are all laughing, but they are cheap(I have six) and you wont break it. They were also used on mk2 Jaguars, so you can get an LSD for peanuts. My old capri mk1 uses one, without LSD, 0.5 inches narrower than standard MK1 atlas, and i have 295 BHP at the wheels!! I have enough torque to pull down a tower block, and have been using this axle for 4 years. You would have to change the mounts to leaf springs, if thats what you are using, and have the hubs/drums redrilled to Ford 4.25 inch PCD from triumph 4.5" pcd. But hey the axles gonna cost £150 max!! O and the prop flange matches, but needs shortening slightly(mine cost £35) by about 1".
Cheerts doug