The other thing about GTF is that the sidewalls are usually thicker. Thinner is better for this purpose, in my experience.
I have been fidding with calipers and the templates and my hunch is that the 10105/10102 is likely to be a slightly better fit. I have a couple of 870 Wingmaster pads (that's what those numbers are) in the basement and am going to check this.
Edit: I think you're right - the 1300 pad is a better fit vertically. The 870 WM pad ain't a bad fit, by the way, if it's carefully centered. It's just a little thin and a little short, not very noticeable for a working gun. The Win 1300 may be better, however.
If it works, it will be a much better installation with an adapter made from 1/8" sheet aluminum. Grind to fit inside the stock, drill to match the SLP's screwholes, then drill and tap to match the pad's screwholes.
I'll let you know what I find, assuming I actually get this done.