[BenI]

2 ideas:

Did you clean up the heat exchanger ends with a file?

I found that when using the 'correct' copper stud nuts, that since they are soft they can catch on the flange and torquing correctly becomes difficult. I always install stainless washers.

PS-I dont know your technique, but I wrap scotch tape around the riser pipe ends on the heat exchanger to tape the gaskets in place. Of course you dont want tape on the meeting surfaces, but this really holds them in place.