Mike

I see there is a scope base mount screw just in front of the aperture sight and I'm assuming there will be a couple drilled and tapped into the receiver. If you use a Weaver bridge mount (or other low profile brand) you should be able to mount a scope while leaving your aperture sight in place as a vee type Weaver bridge mount will allow you to sight thru the base when the scope is removed.

One screw attaching the base at the rear will be adequate. I have used a similar set up on my Oberdorf Mauser 404 when setting up a base from the receiver to the rear of the Safari sight by using a single screw for the Weaver base at that point. (extended eye relief scope). Your Whitworth bolt handle is set up for a scope and with rings to clear the aperture sight you will have the best of both worlds.

I would die for an aperture like you have but still like the option of a scope too.