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I think the first thing to consider is how you intend to stage the car. If you shallow stage a Falcon as much as possible, you should get about 15 inches of rollout before the timing system registers that your car is moving. That way I'd say you're likely to get a 1/4 mile ET on the time slip that's around 2 to 4 tenths under the true time, and the reaction time it shows should be the equivalent amount over the true time.

If that seems like cheating, keep in mind that others do the same thing and at least you've got an ET that can be compared to their times.

On the subject of the launch, unless a powerful car like yours has very good tyres and is on a sticky enough track, I'd run with just enough petrol in the tank to avoid fuel surge, I'd remove all unnecessary load in front of the back wheels and I'd carry some weight in the boot as far back as possible to stop or greatly reduce wheelspin. With my FG XR6T Auto, around 15 - 20 kg's behind the back wheels can make a noticeable difference, although you might need more than that. Of course additional weight will slow a cars potential 1/4 mile performance, but when grip is a problem, I think you're likely to gain a lot more than you'd lose by adding weight.

I've got a GPS based Performance Box (that Motoring magazines use for their acceleration testing) and it's accurate software has shown that the 1/4 mile ET difference (to my car) between a no wheelspin 1300 rpm stall launch and a no wheelspin launch with around 22-2300 revs, can be as much as 0.44 sec. So that's nearly half a second off the 1/4 time and it would take a lot of additional weight to slow the cars potential 1/4 time down by half a second. Just check some of the 1/4 mile ET calculators on the Net.
With your car being modified though, I don't know if a 2300 rpm launch would be safe for the driveline/transmission. Something you might want to check on, I've heard of someone twisting a tailshaft when the handbrake was used to stop the wheels from turning.

Some other points are, don't abuse the car before the launch, or stall it for longer than necessary, try to keep enough pressure on the brakes to stop the back wheels from turning because that can reduce power in the run that follows. I wouldn't drive the car manually, you'd be very lucky to time the changes correctly.
When I took my car to WSID last year I gave the transmission a good workout with a number of full throttle kickdowns just before getting to the track to make sure that it would automatically change at maximum revs. Of course I don't know how your modified car would behave, but my GPS Data showed that it used just about every rev.
As for fuel, I understand that the important MOTOR OCTANE NUMBER of 94 or 95 RON E10 fuel, is the same or nearly the same as normal 91 RON fuel, so unless your car is tuned for these fuels, I'd use at least 95 Premium or more likely 98 Premium fuel.
My XR6 T (still untuned) loses 10% of it's pace on 91 fuel.
I don't understand why people take spare tyres out of the boots of powerful cars before running them at Dragstrips that often don't have a lot of grip. My GPS data has demonstrated that about 85% of the time lost to wheelspin in the 0 to 60 km/h part of a 1/4 run, will typically flow into the 1/4 mile time. ie. 6 tenths slower to 60 km/h, about 1/2 a second slower 1/4 mile ET.
If you just can't make a first gear launch work you might have to use second gear. I got a 12.731 timeslip with the car being launched locked in second gear.