John, the lens projects a cone of light. You're proposing to put an obstruction -- the SLR's gate -- between the back of the lens and the film plane. Draw a sketch and see whether a ray from, for convenience, the center of the back of the lens to the corner of the film will clear the SLR's gate. If it will, you're ok. If not, you can't get full coverage with the lens.

FWIW, I use a number of lenses, from 4.75" to 480 mm, front-mounted on a Copal #0 on my little Graphics. All cover 2x3 just fine. 4x5 may be another matter, but that's for you to find out. Although your idea looks hare-brained it might work for 4x5. On general principles, 8x10 is iffy.

You'd do better to start with the thinnest 35 mm body possible (screw-mount Leica or clone) or, better yet, extract the shutter assembly and use just it.

You might do even better to use an ex-oscilloscope camera Ilex Universal or Alphax. Now that I think of it, Betax #5s aren't that expensive and they're huge.

Remember that for LF long timed exposures are much more useful than short ones. That's why I use the Copal with a front-mounted lens on my 2x3 Speed. The Speed's focal plane shutter's longest timed speed is 1/30, and that's often faster than I want. The Copal times down to 1/1.

Good luck, have fun,

Dan