1980 Dodge Mirada Build Advice

I recently bought my 1980 Mirada J-Body with a 225 slant 6. This is my first (and last) project car. Just looking for some guidance and opinions on where to begin to make it what I want. A reliable custom cruiser with some balls! Not an everyday driver, but definitely more than just weekend cruising.
Is the best bet to completely rebuild the 225 slant 6 with beefier parts for more HP? If so what are the trans options?
Or does anyone offer a new engine & trans combo that replaces the slant 6...

1980 Dodge Mirada Build Advice
Author: 302My-Rada