There are a couple of things you need to consider, Tex...
1) A turbocrazy will do about 1# at an acceptable rate if the stircrazy unit is not heated, and about 1.5# if the stircrazy heat is left on (of course you can go higher, but the rate will be too slow). If your stirring mechanism is not heated, your heat transfer to the beans might be too slow.
2) I would seriously insulate the bottom if I was taking your route for the above reason. Ceramic fiberboard insulation will help contain the heat and greatly increase your potential load.
3) Turbo ovens are not created equal. While many of them may have a 500?F setting, to actually get there it takes a balance of how much air is flowing through the system (not recirculating), true 120V power supply, and in some cases how new the oven is. The Sunpentown analog model seems to have more oomph for the first 20 or 30 hours, then tapers off to a slightly lower level. Some Galloping Gourmet ovens are beasts compared to others. The backpressure can have some effect, as well. You will also need to blow the thing out frequently with compressed air to keep it working efficiently if you are after a larger roast payload.
4) Removing the thermostat and thermal fuse in the turbo oven might be necessary for such a large payload.