/6 rebuild advice

I just purchased for relatively cheap, a 2 door, 72 Dart /6 255, automatic 3. Car had been sitting for years before I got to it, supposedly :BangHead: oil looks good, brakes work good, no ticking, dies at a stop sign (what ever), points not electric, and is a vacuum nightmare, front end needs everything of coarse ......all things I can do....

Looks like the rear main seal is leaking which I am hearing is a small project.... easier to remove the engine to fix...... and taking the car for the longest drive I've done yet, it over heated....... cooled off fast, and while topping it off with water to get home I noticed it had a milky, foamy, substance ? in the radiator neck .....

I have a 318 sitting in my garage.... but.... knowing swaps aren't always/ever direct swaps/easy, would it be CHEAPER/easier to rebuild the 6 or put the 318 engine in, in its place?

My first /6 car..... I only want a reliable fun driver, nothing hot......

Thanks in advance.

Any solid or recommended mechanics in the Thousand Oaks / Camarillo areas in SoCal would be nice.