Hmm -- well the best approach would likely be to do a sort of optical bench experiment and try to come up with the distance between the lens reference (mounting) surface and a piece of white mat board or foamcore to project an image. The subject you point it toward can be 100s of yards away (the moon? a practical infinity) or closer in if you wish to optimize for closer focus. You could then measure from the lens board surface that the lens mounts on to the surface of a dummy film in a holder with the dark side out.
The practicality of pinhole is that pinhole to film distance isn't particularly critical. If I were building a camera to use with a lens I would probably try to come up with some sort of adjustable positioning for focus. Especially with 8x10, there is a fair amount of motion required to focus at things a few feet away. The telescoping box sections mentioned upthread is certainly one way to get there.
It would also be cool to make a frame that matches the film holder, but with a ground glass surface mounted in it such that the ground surface is at the distance corresponding to the film plane of a normal holder. With that and a dark cloth (old jacket, or whatever) over the whole affair, one could adjust composition and focus on the ground glass -- just like a "real" view camera.
You can make a ground glass by wet sanding a piece of acrylic glazing with wet-or-dry sandpaper. It won't likely compete with a commercial one, but with patience it could be usable.
So that's my 1.3 (after tax) cents!