will an OD unit take it

The likely answer to your questions are mostly "maybe." I don't have any personal experience with the OD trans, but I have a '71 340 and a '71 A833 in my Duster. I've heard both sides with people running these with lots of power, no problem. They were after all, installed in alot of trucks. The other story I heard, but not seen any real evidence of yet, is that the alum case is weaker and prone to breakage under higher stress. Heck, anything will break under enough stress, so...

As far as the gear ratios are concerned, the big jump is between 3rd and 4th. Depending on your rear gear, tire size, etc. that may or may not come into play at the track. I can't confirm the flywheel, but that is a definite maybe. You didn't say what the OD kit came out of, assuming a SBv8, but they appear behind /6s, so better check it. I'd go for it and sort out the details later. This stuff is getting harder and harder to come by.