Requesting cooling/radiator/fan help for a slant six

I think your stock rad has an excellent shot of cooling this enough; /6 rads typically had plenty of capacity. Blocking them in winter with cardboard is not uncommon just to keep enough heat in.
Plus:
- the /6 block sides shed a lot more heat directly into the air than the V8's
- your Champion radiator in nothing particularly great.
- your HP level is not all that high
So if you want to do the MB fan, then I think you will be OK.