Hi Butch, some of this may seem a bit basic....just a check list of my experiences really..
The only time I had a overheating problem with my Meadows that was not caused by a defect, was when I held on too long to a gear on long climbs, a bit of steam would escape, changing down increased the fan speed and very quickly stopped it.
I found it best not to drive it like a Diesel, I know it hurts but keep the revs up a bit.....
I agree with Richard's point about the thermostats, they do more than shutting off the rad.
I had new thermostats fitted and the later water system parts, I think they're even more necessary with modern fuel.
Have you rebuilt the radiator cap? The spring can rust badly and so lower the pressure, worth a look if you haven't, it should click quite positively when you turn it close it
Check that the belts are exactly matched as any difference will not be too obvious but could cause slippage, as could a misaligned compressor.
If you haven't already done it increase your anti freeze to 50% to lower the boiling point.
An internally cracked or rusted exhaust manifold can cause symptoms similar to boiling water, so will pin hole corrosion in the passages between the exhaust valve and the inlet manifold hot spots......... trust me I know, been there done both of them!
Check that you have the ignition timed on the correct flywheel mark, there are two, different for mag or coil.
You could also try screwing a permanent pressure gauge into a spare connection to check the water pressure.
Keep up the good work mate, Scammell fans need you...:thumbsup: