Trying to save a ‘66 Barracuda

i agree with MO, and suggest seeing if you can run down a core support. exhausting that option, i suppose you could start with the support out of a 70's car and hack it up enough to get you where you need be.

to what extent is it cut out? just the middle or end to end?

the 904 remained essentially unchanged from 68 on up and even into the 999/998 variants there is compatibility, so besides getting the mount right it'll bolt in. because of the wheelbase difference, you're probably looking at having one made. but you could always pick one up and at least be ahead with the yokes and just have it shortened.