Ritterview, I am excited by your 68s and the new bike, particularly cranks. Post pictures, specs etc please.
I don't think you will regret the 68s - I think you will appreciate the extra aero and won't notice the mass.
As an anecdote I am roughly as fast as my club mates on the flat in a paceline, but downhill the 38s allow me to sail past whenever the paceline hits 40mph down any hill compared with people on box section rims. On the tandem the 68s will be
You will need good brakes!
On my single bike I conservatively went for 20, 24h and they have no sign of flex, even in uphill efforts. I haven't tried them on the tandem, but given some people are riding American Classic CR420s (which btw I think is nuts) I will give the front wheel a try at the weekend if I have time.