low mile 318 how to build?

Chuck, I believe that you are correct.

If you are not changing piston or rod weight... or anything that can effect the factory internal balancing, it should be fine.

I did an automatic to manual transmission swap in a Duster over ten years ago... all I did was pull the flex plate and installed a flywheel. Been running smooth and strong for all these years!

Now... a 360 is a whole different animal since it is externally balanced... but they do make flywheels for that purpose... they are the same flywheel as the 318, with a series of recesses drilled into it to strategically remove the proper amount of weight to maintain the balance characteristics.

The key is the weight of the individual parts of the rotating mass... if you change any of those items to parts that do not weigh the same, then you have problems!