The rebuild - Doh!

Well got the body off the old V8 and started stripping the chassis and body hoping to get the chassis galvanised. Anyway on stripping the body to see all what it needs I could do with both front inner arches done. From what iv read to fit them you line them up with the chassis before you weld them up to make sure its all correct!!! Well thats going to be difficult as the body is in the garage and the chassis is next to it! Whats the best way to get everything spot on without taking the datum from the chassis? I want to get both part perfect before they meet up again and go crazy with lots of paint and rust prevention measures.