Edited on 2015-04-05 15:21:37
Your jag is a unit body, with separate rear and front suspension. The rear suspension is independent with in-board disc brakes.
I would think it would take some doing to adapt that rear end to your Triumph. The older jags simply had a solid axle and may be easier to deal with.
The 3.8 Jag engine didn't have wet liners, if the engine was worn, one would over bore it to 40 thou. over (maximum) and after that, the machine shop would press in dry liners to bring it back to spec.
You could do this with your Triumph as well, the wet liners can be bored out to accept dry liners or over sized pistons. It's much more practical to do that rather then replace the entire wet liner.
I found it easier to have the machine shop do the work rather then trying to source old-new stock.
When I did my Bentley they bored it out and pressed in dry liners bringing back it back to the original 3.5 inch bore. I could have bought new factory pistons, but the technology and materials were old fashioned, my machine shop had me send one of my original pistons down to Ross Racing pistons in California. There they used modern material and technology to create some great pistons that were far superior to the originals. They were forged aluminum alloy which were made out of round and when they warmed up, they became round. The skirt was very short as in a modern engine as apposed to the very deep skirt of the originals. With the skirt being short, there is less friction on the bore and less heat produced, better for an old engine with a long stroke and antiquated water pump.
You could apply this to your Triumph engine of you decide to rebuild it.
The Jag has a better engine, it's of a more modern design then what's in the Triumph, so keep that in mind. That Jag engine design didn't really change that much until Ford took over and it was finally retired. It was ahead of it's time for many years.