I have done it two different ways, neither according to VAN's plans.
On my RV6, the cooler was mounted on the left front baffle floor. Cooling was superb. The drawback was longer oil line runs. You also have to beef up the baffle to accept the cooler or you will get cracking. (I backed up the baffle floor with .063 and it held up fine.)
My RV8 has the cooler on the firewall. I have a 4" SCAT from the rear of the right baffle. Cooling is exceptional. Even on hot Texas summer days, I rarely see oil temps above 210, even in the climb. Normal cruise temps are in the 170s, so I would actually like to have a little higher temps, so I plan to partially cover the inlet to see what happens. I would use 3" SCAT if I did it again. (My engine is ECI IO 360, 180 HP.) I have a small plenum over the oil cooler that holds the end of the SCAT about 3 inches off of the cooler, which I think helps.
I have seen quite a few installations on the rear of the left baffle, according to VAN's plans. I have also seen many of these with cracks on the baffles.
For me, the firewall installation is straightforward and easy and does a great job of cooling. With 180 hrs since the installation, no problems at all.
Good luck on the method you choose.