I once built up an FE and didn't have one of those baffles, so it got put together wihtout one. It was fine without it. In fact, I think most other V8's don't use one.

If it were me, and I forgot to install one, I wouldn't worry about it, but I would save it and install it next time I had something aprt. Especially if I knew my exhaust crossover was blocked. Those FE manifolds are a pain to remove, wiht having to take all opushrods out, etc.