Dan, sorry, I didn't see you had posted this question til just now.
I am using a Bendtsen adapter to mate my 394 to a Chevy TH350 transmisson. I had considered using the same Rotomatic (or whatever they are called) Olds transmission, but there were a couple of reasons I didn't. First of all, it is cast iron and weighs as much as the motor, and secondly, I like to drive my cars far from home. I could just see me getting 500 miles away and having the transmission go out, and shops today probably don't know how to work on one of those old ones.
I was getting quotes locally around $ 1700 to rebuild the stock Olds transmission, and for that money I was able to buy the adapter and a built TH350 that any shop in the country can repair.
It does take a little bit of doing though. The Olds has a goofy engine mounting system with one mount on the motor and two on the bellhousing. I had to make a new mount for the front with two mounts on it, and then a second pair of mounts on the bellhousing, and one on the tailshaft of the transmission. If I had not done it that way there would have been too much strain on the bellhousing due to the long distance between the front mounts and the one on the tailshaft.
Hope that helps.
Don
Here is my motor and transmission mated, but before the mounts were made.