That would have been my approach TBH - camera on tripod, frame up the shot (with or without the shadows being cast), tape measure to ascertain the exact distance to the wall, set to that manually on the lens, THEN start playing about with getting the shadow as sharp as possible... Then again, It seems these days I pretty much always approach set-up shots with step 1 being "get the camera on the tripod..."
I did the setting up [shadows] first, as in the actual physical set up, then played with the digital cameras on tripod to get some idea of settings, and positioning. Like I say, that is far more difficult than you might initially imagine - ok, more then I initially imagined. To get a really crisp shadow, the object has to be close to the background you are casting the shadow onto, except of course, bearing in mind the size of the objects in this case, and the fact I only had about 5 x 4ft of clear wall to play with, the problems was balancing a sharp enough shadow without the guitars intruding into frame. The original album cover is a painting, so the logistics were easier
I checked what distance I had on same lens mounted on the digital using AF and making sure the image was sharp [or specifically, the wall was sharp], then transferred lens to the film camera, being careful not to move the tripod whilst swapping, set manual focus and applied same distance. So, I did several shots did several shots tweaked just marginally either side of this to account for any slight differences in focal plane distance, as well as several tweaked either side on exposure settings, just to accommodate for the fact I can't get ISO160 on the digital, and bingo, almost half a roll gone. The whole thing is only lit by a 60watt bulb, at night [to prevent other shadows I didn't want] so pretty dim through a view finder. I had expected to have to have to do this, and in some ways is why I like these challenges, just the 'having to think things through, plan, then change ideas on the hoof when something does/doesn't work, etc.