Be prepared and safe with an Interlock kit for your existing panel
If you have an FPE panel you could be out of luck. I looked high and low. You can change the guts to a newer brand. You will have trouble with your insurance comp. anyway. "they don't like FPE" not only are the breakers not accurate,  the panel dimensions are too small also. 
You can Use wires that already go from your panel to your AC condenser. In Florida 30 amp is common. You would have to move the 30 amp breaker that feeds your condenser to the position that your interlock calls for. If you use an existing wire, like your condenser then you wouldn't be able to use the condenser when the interlock was installed.
The simple solution is to build it, test it. Then remove the interlock kit. Reinstall it if you need to. The whole standby is not in use most of the time. So why add wire, and expense. and sometimes you don't have room in your panel for another breaker. Let's say you need to add a welding circuit outside, Some are 30 amp and some are 50 amp, so I would run #6 wire "50 amp" wire so that if someday I get a 50 amp generator, the wire would handle it. you could still use a 30 amp breaker and a 30 amp plug if that's what you need. And again you would use it as the welding circuit for welding. Power fails. reinstall your interlock. If you need to weld just plug the welder in the gen.
     Before you change or update your electric panel. consider some of the new systems. They will turn off breakers. let's say you're going to bake a turkey the new system would turn off your AC when the oven is on. and would turn it back on when your done cooking. This also lets you get smaller gen, smaller fuel tank or more run time on the tank you have room for. Sometimes you have trouble with AC equipment, The AC turns on and expects to have what it needs, but the gen is just idling. Not making the output that is on the label, until it has had time to rev-up the engine. And in that much time, you could fry your AC. But they do make a hard start kit for your AC. but it's wise to ask your AC man before you run the AC on a small gen whos max is close to the demand of the AC  If you need to run the AC you could get the plug for the gen and the other end you could just put the wires in the same place as the wires in the AC disconnect. But the danger with that is the end that plugs into the gen is a male so if you removed the interlock but left the cord going to the gen then it would be hot 220v exposed. or it would blow up your gen. Lets not look at this stuff as rocket-science. But it is dangerous. Handling a gun is simple too but the mistakes are fatal. The more moronproofing you require the more expensive it is