Oh wow, they got to use a 75 footer down that line for once? That's good, but as you say, there can't be an oncoming train otherwise they wouldn't fit together, although technically, 75-feet cars could use that line on condition that no other train crosses paths with the 75-footer.
It was all well and good for a fantrip, but they could never have to have that worry for regular service. With current safety restrictions, they'd probably never even allow a fantrip with that equipment on the JMZL lines again unfortunately. 1976 was probably the one and only time that the 75 foot cars would ever visit there. I have seen photos from this trip.
Oh Well, maybe they could be used on the M and L then on a trial run, but I guess they'll have to straighten the J line curves out, although the NYCT doesn't have the millions to do that retrofit.
Forgetting what I just said above, it "could" be done on the M however unlikely.
The L also has places where it would have to be retrofitted. The curves around Graham and Grand St can not handle the 75 foot cars.