I agree, and if you factor in re aligning the doors to get a decent gap it can take days.
That`s the problem with hand built cars.
There are a few tricks that can be utilised to make it slightly easier, I always fit a good fitting spacer block to the very front of the frame to help clamp it firmly to the fibreglass, along with CS allen screws, to allow the door card to fit better.
Reliant left it open and used large button head screws, which if over-tightened splayed out the frame and left it loose.
I also use SS clamp plates with tapped holes to bolt everything back in place, fit and forget.