As no one else has responded I will try to help.
I was surprised to find that Myfords supply the gib strip with dimples already machined regardless of the number of adjustment screws. As you say, they seem to list only a four-screw item which I think is for the standard length cross slide. You seem to have the desirable longer slide.
I have one like this which is, as you say, 8.5" long, It has six adjustment screws at 1.625" centres and I have added two lock screws and two dowels to prevent relative movement between gib and slide. A gib to suit this would be easy enough to make from mild steel strip with its edges milled or filed to correct angle. It would then be clamped in position and marked for drilling the dimples, etc.
This kind of work is covered by G H Thomas in his Workshop Manual which I suggest would be a better use of your money.
Good luck!