SMPS (LM2678) what transformer to use and why?
You are confusing a mains powered SMPS with a Buck converter.

The LM2678 is a buck converter, it uses an inductor but no transformer.

This is a typical implementation of a DCDC converter using the LM2678:



Note that there is no transformer. The inductor is the component marked "220" (for 22 uH).

For the LM2678 to operate you need a DC voltage up to 40 V. You can make that 40 V (or lower) form 240 V AC Mains voltage using a bulky 50 Hz transformer like you have done sofar.

Or you can use a Switched Mode Power Supply (SMPS) which could look like this: 


Note that this has a (small) transformer like the one from your question.