If I read that site correctly Paul their 1" belts only start at 46" long up to 59". At 39" Dave has no further adjustment, even allowing for the extra 1" taking up a bit of play I think 46 would be much too long . This has been a constant problem for both my 37 and 38 cars, they have wide pulleys and these belts from All Cads are no good at all.
Jim Stamper has very kindly been trying to obtain one in your country for me with no success so he and I both run with the 7/8" which sadly sits in the bottom of the pulley and has the tendency to screech if not adjusted very tight, not a lot of good for the bearings, The earlier cars actually should have 1 1/8".
So if theres a solution to this I would love to know about it. Here in the UK Imperial sizes are no longer available. This subject has been covered before on this forum, it would seen changing to post war pulleys is one answer but the belt adjuster is much shorter.