There was a factory block heater installed in one of my vintage mustangs, which I removed when I rebuilt the engine. It had an element that went in through one of the freeze plugs, and the heater element was held in place with something that looked like a little screw type wall anchor. There was a spring loaded backing that went inside the block and popped open when inside, and a screw went in through the plug. When you tightened the screw it pulled against the backing and pulled the plug into the hole, sealing it. I've heard stories though of these things rusting through or just falling out, so I'm a little leary of them. I'm not sure where the freeze plugs are located on the 850 motor, but being it's transverse mounted I imagine where ever they are it ain't gonna be easy to get to the them. probably access from the bottom would be easiest, from the top would require you to remove the intake manifold, or, if on the other side, the exhaust manifold. Easiest way to get one of those plugs out is a screw in type dent puller with slide hammer, but I doubt you'll have room for one of those in there.

I also had a block heater that I used on one car that was magnetic and just stuck onto the bottom or side of the oil pan. Easiest thing to install, just reach under and stick in on, and it heated decently enough. However, being that the 850 oil pan is aluminum this obviously won't work.