When you want perfection, you have to go to extreme measures. They boy's decied to fabricate a jig to sit the car on, just so they can make sure that the chassis rails are nice and straight. Fitting the running gear then finding out that it's not true would be a nighmare, plus extra work.
The jig will also take out any twist that the chassis may be through years of road use. Not long now until the engine and gearbox can be fitted, as well as front and rear suspension.