/6 rebuild advice

SoCal mopar said
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.

#1 Do a compression check. All cylinders fairly even? Good. Move on to #2 If you've got a wonky pressure number, do a leak down test. Air will go one of three places: Out the exhaust, up through the carburetor or up through the rad. If it's intake or exhaust, you have a bad valve and need to pull the head. If you have bubbles in the rad, it's a head gasket and the head needs to come off.

#2. Do a pressure test on the cooling system. Hold pressure? Good. Milky oil is probably condensation. Not hold pressure? Probably a blown head gasket. Fairly easy fix. Little known fact. Chrysler was notorious for milling cylinder heads flat and not bothering to have the combustion chamber volumes in spec. Take the head in to check the valves, guides and measure the two end combustion chambers for volume. If it needs valves and/or guides, do that first, then check volumes. I pulled a stock head off of my '79 225-2V and milled .030" off it to JUST bring it into spec. Taking .100" off will get you about 9.0:1 compression.