You know this is multi-faceted. One thing is absolutely certain is that you cannot supply your panel box with electricity from your generator unless you have the proper switch installed. This is for the safety of the electrical crew working on the line. A back feed from you could kill them. In most states it is charged as murder. So, now to really answer your question
I would probably do something like this:
From the blower/igniter of your heater you have wire going to your fuse box.
Now, I am going to assume you have some working knowledge of electricity.
I would mount a 110 box in a spot near the heater. The wire would go from this duplex receptacle to your breaker box. In essence you have just added a receptacle box. Next, I would put a plug on the wire coming from your heater. Normally, this is plugged into the receptacle you just mounted.
Next, I would mount another receptacle box near the first one. Run a wire from this box (Iprefer #12) to a length that will reach your generator outside. Put a plug on the end. So now, your power goes out in the middle of winter. You start your generator up and plug your cord in. Since the other end is hooked into the receptacle box there are no hot wires hanging around. So you unplug your heater from the house supply and plug it into your generator receptacle and you have heat. Now you can run an extension cord from the other side of this outlet and you have lights.
Do your math before you need it. See how many amps your motor heater draws, and while you are at it, check your refrigerator, and anything else you think you might want plugged in. Remember, if your generator is 8000 watts it funtions at about 80%, or 6400 watts. Most generators would handle your original request of blower motor and lights, but if you want more make sure you know what your load it.