Just been having a think - dangerous I know! Having just had a quick play in solidworks, it would seem like you could quite easily get two identical plates cut to use as jigs, one clamps to the front frame. The other would then be positioned 13mm behind rails U1 and U2. 4 bits of studding later and a few little spacers and you then have a guaranteed positional relationship between the mounts and you can cut up the jig plates to use to make bits of the car out of
Anyone tried going down this route - it would seem far too easy!