Hi Spook,
Not sure if you know but the belt numbering system is actually fairly easy to interpret.
The part number you quote "SPK1875" should probably read 5PK meaning 5 rib. 4PK is 4 rib, 6pk is 6 rib etc. The following number is the length 1875mm
To work out the belt that will suit your application you just need to get a length of webbing, a tie down strap works well. Wrap it around the path the belt would take and mark it for length with the tensioner in the relaxed and compressed positions. Then just roll out the tie down strap and measure the length to the two marks. Then order a belt that is somewhere between the two.
The belts seem to go up in length of 5mm increments. Once you know the length and the number of ribs you can punch that into the Gates web page and get a bunch of cross reference part numbers or at least see if that size is available.
Gates Search
Bearing shops that stock gates will have universal belts available that you can just order based on that part number i.e 6PK1995 (6 Rib belt 1995mm long)
You can also search through some of their online catalogs and you might find the vehicle that belt suits. Buying them from automotive shops can often work out cheaper as the bearing supplies can charge a premium.
Gates Automotive cataligue
Hope this helps
Cheers