What do you think would win in a race?
I'll take that bet. Give him the 5 car lengths, I'll take you on your bike all day long, even if that Mustang will do a 1/4 in 13 flat (It won't). Even with 5 car lengths, you will still kill him. 5 car lengths at 100 mph = 0.51 second (100 mph = 147 feet per second, your car is 15 feet long, 5 lengths = 75 feet, 75/147 = 0.5102) so he's spotting himself 0.51 second. He'd need a minimum of 3 times that to even be close. You'll be by him before you get out of first gear. Even if you granny shift, you'll still beat him so bad you could stop and eat a sandwich at the finish line before he got there. I'm guessing he won't even crack into the 14's with that car. Brand new stock they were only good for 14.8 with a professional driver. The mods your buddy did on it made it louder, not faster, plus it's now 22 years old.
Re-reading your question, does he want to start 5 lengths behind you and have you launch when he's even with you? If that's the case, you'll still kill him. He might be going 30 or so (?) across the line. A car that heavy won't be going that fast in 75 feet. If anything, it's better for you if he starts from behind like that. Either way, your bike will be going by him as you shift into 2nd.