As others have said, pretty much all of the value of digital camera is in the back. The front/top basically is for mounting the lens, but also contains some useful features like the shutter button.
Let's think how a digital back is going to interface with the film mechanics: no doubt some kind of mechanism could detect the shutter press and/or film winding, but it all sounds a bit clunky and you've still to to wind the 'film' to cock the shutter. Digital cameras just do this and I doubt a 35mm digital back containing these special mechanisms (and especially with such low expected production runs) could be made any cheaper than a low or even mid-range digital.
The real value of your old stuff is the lenses. I've got a reasonable selection of older Nikon AI lenses (and a couple of F3 bodies) and I use them with a D7000 which cost about double your 'few hundred dollars' desired price. Works great (the APS format is very usable but you can get full frame bodies as well, e.g. D750) and also takes very nice looking videos. I can't see the F3s ever getting used again - even if they suddenly turned into 'digital' camera (that motor drive is pretty hefty and I wouldn't want to bother with winding again).