I would more or less take Ozzy's post as a good guide
Anything built out of tube & has a mulititude of welded joints is going to be subject to a certain amount of distortion.
All depends on how far out of square you are really, is it the seatback area out of square, front frame out of square or a combination of both. ?
From a suspension point of view.
As long as the front frame & dif cage are good and square it should be possible to fit the suspension brackets in the right place.
The correct fixing of the suspension brackets is critical to good suspension & wheel geometery.
From a bodywork point of view.
Depends on the route you are taking.
An alloy rear tub & sides are quite forgiving as they can be tailor made to suit.
GRP will fex a few mm to counter a small amount of out of square.
Seem to remember if the GRP sides & nosecone need to have the chassis/front frame the right height as they may be tricky to fit.
Sorry it's all a bit vauge but unless we know how far out of square you are it's a bit difficult to offer the right advice.