Only ever did one drift on the Lehigh, in early spring. From JT to Bowmanstown. I cant speak specifically about a fall float.
I did fish the river wading in the gorge section a few times in early November. Pretty rough sledding. The river can be very finicky when water temps get below 50. Not your best fishing. If this warm spell hangs around abit, you may be ok. Keep an eye on USGS water data for temps and flow.
That said, a float is a pretty cool experience. A good chance to learn a good swath of the river in a few hours.
I wouldnt worry too much about redds.
You dont get to wade very far from shore and walking up and down mid stream is a fools errand. Most of the river, above JT anyway, is comprised of bowling ball size and larger rocks. Not much spawning habitat.
I may catch alot of hell for this but its my belief that most of the breeding is done in the tribs.