For what it is worth, here is a writeup I did a while back. Hope it helps.
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GP 1/3 Pitts and OS Pegasus FF-320 4 cylinder

After reading a review in Model Airplane News about the Great Planes 1/3 scale Pitts my
interest was tweaked, especially about using standard servos. Then I read somewhere about a FF-320 being installed in the 1/3 Pitts. Both of these things caused me to purchase the Pitts as I had already owned the FF-320 for about 7 years. The 1/3 scale Space Walker kit that I had bought for the FF-320 was still in the box and no immediate hope that it would be built.

I have finally finished the GP 1/3 Pitts ARF with the OS Pegasus FF-320 4 cylinder
engine and this will be a report of what I did.

The first problem I encountered was there where no holes marked on the lower wing for
the I struts. After several emails and two phone calls I was able to get the dimensions from GP.

The engine installation required the firewall to be moved back 3/4 of an inch. This was
done by carefully cutting out the old firewall and making a new one from two pieces of 1/4 inch aircraft plywood glued together to form a 1/2 inch thick firewall. This firewall was dowel pinned to the original box using bamboo. To maintain alignment of the built in right thrust I made a jig from 1/4 inch all thread rod and a plywood ring to center the prop hole in the cowling.

Since I was usin a 3.2 cubic inch glow engine I figured that it would use about 3 ounces of fuel per minute. The 24 ounce tank supplied would only last about 8 minutes. I decided to install two tanks. Since the carburator is on the bottom of the FF-320 the fuel tanks had to be moved down to the bottom of the fuselage. Two of the 24 ounce tanks would fit side by side. I contacted GP by email and phone calls several times over the course of 6 months and the tank they send with the plane is not available separate. I finally used a 16 oz. tank as the second one.

The MAN article said that standard servos were used. However, in the next issue that was corrected by the author and he listed the actual servos that were used. After analyzing the ones he used I determined that standard servos should work. Where he used one servo on two ailerons I chose to use one servo on each aileron. This means that two 44 in oz. servos will do the work of one 88 in oz. servo. I used four Futaba 3004 servos on the ailerons and two on the elevator. I did use a Hitec 605 on the rudder as it also takes the beating from the tail wheel. For the throttle a Hitec 81 was used. Before you say these servos won't work keep in mind this is not a 3D machine. It rolls, loops, spins and snaps well.

I built my own on board glow driver that has two isolated circuits that controls two plugs
each in series. This glow driver allows control via throttle position or can be turned completely on or off on demand by an auxiliary channel.

The receiver installation is straight forward with exception. I believe that batteries and
switches are the most common failures so I use Red Schoefields recommendation of two battery packs and two switches plugged in to two separate sockets.

Due to the heavy engine and on board glow driver, this plane has weighed in at 20.9 pounds empty. Slightly heavier than the ones I have been reading about.

I have the plane balanced at the 5 3/8 inch point which is recommended the CG.

With a 22x8 prop the engine runs great and after about 10 flights is turning 6400 static.
It idles at 1000 to 1200 RPM and can be set as low as 720. At about 1/2 throttle the engine sounds just like the full size plane. The exhaust pipes are straight pipes made from 3/8 inch sink faucet supply tubes. They are chrome plated soft copper and are easy to shape. I installed a pressure tap in one of them. To sum it up the sound and low vibration is worth a lot of the cost.

The plane tracks fine on take off and climbs out smartly. It even has a lot of vertical, more than I ever expected. It rolls with very little elevator and tracks like it is on rails.
Now every plane has a dark size. As with any biplane, upon landing you must keep the nose down and speed up. It will full stall (three point) land if your judgement and timing are perfect.