[Steve O]

On my 1964 2600 Sprint, which too did not want to give up its old non-responsive thermostat, I sprayed WD-40 liberally around the threaded area, let that soak for about an hour, then used hefty channel-lock type pliers to carefully thread it out of its long-time home.
Large cooling capacity not withstanding, the engine should run at a 'normal' temp...as you know, that's what the thermostat is there for. Shuttering the radiator should not be necessary to achieve this.
Parts lists show a unique Alfa part number for the 2600 thermostat, which is of course unobtainable. I took the obvious alternative and used a Duetto part (perfect fit, and readily available from IAP, Centerline, etc.), with anti-seize compound on the threads, and my 2600 now operates at the identical 160F winter/spring /summer/or fall -- whether trapped in traffic on the I-15 on a hot August afternoon, or cruising full speed in January. Oil temp follows along, remaining slightly lower unless run for long periods at high speed, (but never goes higher than the coolant temp.)