There are those that have older Jeeps that are equipped with the Buick Nailhead TH400 that uses an adapter to go to an AMC or Buick BOPC bolt patterned engine.

I know of some that would like to replace their tired or broken engine with something else.
It's been a reoccurring discussion on the IFSJA forum.

For example: Tonka200 from the IFSJA forum is working on replacing a Buick Dauntless V8 with a fresh newer Olds 350 V8.
What's the problem you would ask? Buick and Olds share the same BOPC bolt pattern, correct?
When Frank bolted up his nice fresh Olds V8 to the adapter ring and Nailhead TH400, he discovered the starter was on the wrong side.

The standard answer has always been, dump the Nailhead TH400, find a modern TH400 (or other transmission) for the engine you are wanting to use.
Then gut the Nailhead unit for its output shaft and xcase adapter, install it on the newer TH400 and rebuild it while you have it apart.

This answer turns a budget minded and simple project into a full blown expensive and deeply involved project.
IMHO, this answer is not always acceptable.

My suggestion would be to develop an adapter ring replacement to allow a wider selection of engines to mated to a Nailhead TH400.
There are plenty of folks with older Jeeps with Nailhead TH400 that could benefit from such a product.