Thanks Geof. These look good. The one at:
http://www.summitracing.com/parts/MCH-1675/ looks best for me I think. I did not want to use the 'old' Mustang tensioner as I have a billet adjustable one for that.
I am assuming that the smooth water pump pulley would indeed slip with the grooved belt. It may not of course.
At any rate happy to share the postage!
Are you happy for me to order these?
Merv