Drive Train Upgrade Suggestions

A. Welcome to the site
B. Maybe give some ideas as far as your budget goes/time you expect to complete it. These cars can fit just about any motor ranging from your slant 6 just a little warmed over to a really hot stroked small block (people have stuffed monster hemis in A bodys but you said you don't want to go big block so I'm assuming no hemis either)
What are your expectations or limitations

P.S. If you take a look at the photo garages here you can find a lot of inspiration if you aren't even sure what you are looking for now