Wow I've been called a lot of things. Brilliant is NOT one of them LOL...plumb bobs are just an old school measuring trick...more from construction/renovation of homes than from cars.
Just remember your tolerance is 1/8" variance or less. That's not much, so this takes some precise measuring. So be patient and take your time doing this, and be very accurate with your marks and in mirroring the locations of the string at each point on the car. And don't lean on the car accidentally and let it roll 1/4" when you're halfway thru making marks!
In a perfect world, you borrow a tram gauge from a body shop. It's largely an obsolete tool that most good body shops have laying around and one might be willing to loan to ya if you happen to know someone that works there. Most body shops use computer measuring equipment these days for frame/unibody measuring and let their old tram gauges gather dust. They're awesome for precision/comparison measuring while straddling things like driveshafts, exhaust, and all the other garbage that's probably in your way. But you'll need the car up on jackstands to use it for sure.
