I know I am kinda late on this thread and I did not read all the comments, so not sure if what I am about to say has been said before. LOL.
Just about any healthy, sound horse can do LD's. It takes a little more for a 50, but I have seen several different breeds do 50's too. If saying he is built like a TB means he is on the leaner muscled side then that is a good thing. That means his heart rates will be better since he won't have so much muscle to cool off. And my guy had a ewe neck when I first got him. He was under weight also, but as he has been gaining weight and muscle it has slowly been going away. He might always have a little bit of one, but I don't care.
And you could maybe be competitive on him, it just depends on how he handles the distance riding. I have noticed that the different breeds seem to be in the Top Ten more in the 50's and up than the LD's because of the finish on LD's being when your horse's heart rate drops. And though I have seen other breeds drop as fast as Arabians, it is very rare. It's in the Arabs genes to do this, they accel at this sport, so it is hard to compete with them. But like I said I have seen some other breeds do it.
I wish you and your horse the best of luck and hope you have tons of fun!!!