engine rebuild/swap

Hello, I'm new and would like some advice.
I have a 50-something Willys station wagon in restorable shape. I would, however, like a better engine. I am not looking to build a hotrod; instead, I would like to rebuild the engine for as much torque as reasonable possible and still get some reasonable gas MPG. This vehicle will be used as a fishing and hunting vehicle in rough country. I have thought about installing a Chrysler slant 6 and modifying it for more torque. I would also like to know what transmission I could use. Does this sound OK? Why? Why not? I would appreciate any advice you have, especially if you have hands-on experience on a project like this. Thank you for any help you can be.