First a disclaimer: my engineering experience is in aviation, not automaking.
In general steel can be formed a lot more accurately than fiberglass, and that was even truer twenty years ago. If the variation was present at the factory, it's much more likely in the body than the frame. The builder would shim the body so the doors and other stuff lined up, not so the body would lie smack dab on the frame.
The fundamental problem here, when check dimensions don't check - be it on a pranged airplane or a car under repair - is determining what's really bent. The physical features designated as dimensional references might have moved. It can be most trying, even for an expert, to interpret measurements of a damaged structure and arrive at a really correct model of what happened to it. Lots of people, having taken the position of designated reference features as gospel, have twisted up structures something awful trying to make the wrong things line up.
There's more dimensional variation in manufacturing that you might suspect. When something like a perimeter frame for a car is fabricated it's mostly welded with the parts clamped in a jig, and the jig is assumed accurate. (They do get checked periodically, at least in the aviation world.) But tooling wears, clamps wear, components have dimensional variations, all sorts of odd things happen. The two "identical" 747's Boeing fitted out as Air Force Ones were - are - eleven inches different in length, with nothing anywhere being out of spec. The tooling for a frame would be designed to hold the IMPORTANT dimensions closely and let any variation be taken up in other areas - hence the presence of shims and adjusters and such. It's not really needful for all of a car's frame to be spot-on accurate, although I grant it's certainly aesthetic.
I guess what I'm driving at is (a) be very, very careful determining what's really out of position relative to what, (b) be careful also in determining what ought to be fixed. If everything fit on the car as the frame was built, moving things around will not necessarily help.
-- Vandervecken