Hello Steph
I have had many different type of Triumph, couple TR 4's, a Tr-6, and a couple of TR-7's along with the coupe that I am now working on. Each one has had there unique problems. With patience these were worked out.
Like the other have stated, step back a few weeks to consider things. You just spent all that money on a new transmission. It would be a shame to loose all that you have invested, time money and effort on a nice looking TR.
Like the other, I would in my leisure pull the valve cover, see if you do see some thing out of sorts. If not drain the oil and use a magnet to see if there is any metal in the pan. These things don't cost you any thing but may give an idea as what may not be wrong with the engine.
In any case what ever decision you make will be the right one.
Dale
1974 TR6
1977 TR7 FHC