[Paleo Conservative]

Well, I don't think it was ever under consideration for this modification.

I seriously doubt it would have as it would have been an oddball in the C-141B fleet. If it was going to be a uniquely configured plane, it made sense for NASA to have it.

[Paleo Conservative]

Also by not doing the stretch, the C-141 with Kuiper Airborne Observatory could carry more payload (by weight) or more fuel.