Skip wrote:
And, of course, if the 3/8-10mm route
works, that will save me having to buy new pulleys, a route I'm sure
will be horribly expensive as they're no longer made...

Skip,
I may have missed it in past posts, but have you checked carefully to
see that the V-angle on both pullies as well as the belt are all identicle?
Balmar at least has supplied slightly different V angles on pullies
with their alternators to suppliers using the same part numbers. If any
one is different it would eat belts.
Red