I was fortunate enough in my youth to have had the opportunity to see tridacna clams living off the cost of Manly, AU. I'm not at all certain that it would be possible to have 'too much' flow, in comparison to their natural environment.
Hm. This was many years ago, and it wasn't like I stumbled upon a big garden of clams all in rows... just noted several hippopos and gigas clams in and among the reef. There may have been other, smaller ones around, but the big ones were hard to miss. Shallow water, 6-15', turbulent... lots of water movement, but not necessarily in any particular direction. I didn't really note any particular orientation, aside from 'sunny side up'...
I wouldn't worry too much about orienting your clams... They're more mobile than folks think. They'll manage to get themselves oriented, sometimes much to our frustration, as we try to 'arrange' our aquariums.