That engine will cool just fine under a GT body. I own and run
many different on-road .21 engines (Picco, Novarossi, MaxPower,
RB) with the smaller, lighter on-road cooling heads with no problems
in my 1/8 GT cars. If you're that concerned about engine cooling,
cut a hole in the winshield of the body directly in front of the engine
and cut another hole in the rear window directly behind the engine.
Unlike off-road truggys, the 1/8 GT cars get alot of airflow under
the bodies. Sometimes too much air, you'll find out find out when
you get that Picco broken in and get it up above 60 m.p.h..
It's quite a sight seeing a car that heavy, catching air at those speeds.
Takes off like a kite, does a few flips in mid-air and slides a hundred
feet across the tarmac on it's lid.