First a disclaimer: my engineering experience is in aviation, not
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.