I went through this on mine.
I bought both the F-601K-1 recess and the one for the RV-4. The RV-4 recess is completely useless for the RV-3 since the -4 has a far different firewall shape. The F-601K-1 recess was not tall enough so I had to add on to it.
I described this part of my construction iin my blog here on VAF, in posts 241 through 245.
I'm repeating some of the photos below. If you're only adding the governor, the stock recess ought to be fine. Adding the filter changed things. Although it would be heavier, a remote oil filter might be appropriate here.
Here's what I learned for the locations of things:
This is the back of the engine:
The box I built:
It was so handy having the engine on hand before I started the firewall!
One thing that I'm not looking forward to is maintenance with the recess. While Andy Hill assured me that the filter can be changed, I think that both that and cleaning the recess might be difficult.
RV-3B, now skinning the fuselage