Well, there are a few things that need to be considered with this pairing.
First of all, I recall the -6 to have a 180hp limitation. This, in and of itself is not really too important. People have installed 200 hp IO-360's in the past and they are holding together fine. I understand they have cooling issues to contend with.
Compounding this, the 390 has a propensity to run hot to begin with.
The weight of the 200hp angle valve 360 is already heavy on the -6. The 390 is about 8 - 10 pounds heavier than that!
Soooooo, do your W&B calcs conservatively.
You can go fixed pitch and save some weight and complexity.
Question, how much will you pay for the engine? It would have to be less than $25,000 for me to even consider it if I were you, because that is what a new IO-360 Para valve, injected would cost.
...and that is the engine that belongs in there to begin with!
If you end up not wanting the engine after all this deliberation, shoot me a message. I would be interested in using it for my -7.

CJ