Hi Trevor
I am sure you will have done much research but have you taken the old prop in to a prop company to see if they can ascertain what it was changed to? they can only change the pitch by a couple of inches at the most.
Or failing that have you spoken to Volvo to see if they know the boat/engine and what prop size they calculate it to be
When I built Great White I used Tiegnmouth Props, they calculatted what type,size pitch to fit from the boat and engine specs.
On launching I was very pleased to see that they had it 100% correct.
I am not sure if they still trade but I am sure there are many companies around [Lake engineering 01202 621631, Prop revelutions 01202 681662] that may be able to assist
Good luck
Charlie