Diagnosis in the internet is a bit difficult, especially when unidentified noises are concerned, but my question is: what made the guy think it was definitely the main bearings? It is very rare for the main bearings to wear to the extent they can be heard, before the big ends fail. In fact, all the years I've been dealing with these cars, I've never seen it, though I guess there's a first time for everything. Big end bearings are the usual casualty of oil starvation.
Now, a couple of other things with regard to jts engines: camshaft wear is very common, resulting in a noise that can often be heard around the 2000-2500 rpm range. Also, many of these engines make a noise from 2000-3000 rpm which sounds a little like the start of big end failure, but is not. Years ago, when I first heard it, I thought the engine was about to give up, but now, especially considering the age and mileage of most of the cars, I simply consider it "normal for a jts"; a bit of the old "they all do that, sir" syndrome. The problem is knowing what's normal, and what's a warning of an engine about to poo its' bottom end out.