Hardly any of the old 73-79 300 Six's had an Auto trans.
What version of the C-6 will fit the 300 block?
What else would I need?

Why didn't the Six's come with an Auto?

 

The 300, 302, and 351W all had the same bell housing pattern.

 

Very few 300's had the C6 since the six uses a "small-block" bellhousing pattern. Most of the sixes came with C4.

Vampiro here has an '78 with the 300/auto.

 

Try looking for E250 cargo and delivery vans. A lot of them had 300's in them for fuel economy, and auto's for easy driving. The 250's had C-6's in them for durability. You can often find small bell C-6's on eBay.