Firstly, which 390 or 429 motor are we referring to?
If it is the later 429, then they used a TH400 anyway, and I am sure that the Switch-Pitch TH400 can be swapped for a normal TH400.
But, if it is the earlier 390s, then the swap you are talking about isnt a simple bolt-up exercise.
You see, firstly, the bellhousing on the 390 is round, and the later Cadillac is "odd-shaped"
Next, the starter on the 390 bolts to the bellhousing, whereas the Starter for the TH400 bolts up to the block.
The 390 has the ring gear attached to the torque converter, whereas the engines using the TH400 utilise a normal flexplate.
Any Adaptor will have to have the starter mount, as I am not sure if the 390 block can be converted to mount the starter on to the base of the block.
Having said all the above, nothing is impossible, but miracles take longer.
Hope this helps a bit.
The Tassie Devil(le),