I started out (literally, first car I owned) driving Sprites, then moved on to MGBs, Four cylinder TRs, and one big Healey. They were all basically a scaled up Sprite, bigger motors, transmissions and axles, bigger carbs, but all awfully similar. Then I got my TR250. The Triumph IRS has it's own learning curve, and after years of not worrying much about the back end other than the occasional leaky seal, sort of seems like a royal PITA at first. They can work well with proper maintenance and or modification. But they usually need some attention to make right. Sounds like you are moving in the right direction. Have fun.
Also will add that like Madflyer, I fabricated a plate from 1/8" steel for the backside of the frame where the brackets bolt on, seems like cheap insurance.