here is how i fabricated the rear chassis rail to accept the GRP rear tub,
the main dimensions are
long rear box tube = 99cm
the 2 spars from RS14 to the 99cm tube = 17.6cm, set at 5 degree,s upwards.
the 99cm tube is an odd dimension which is to allow the inner radius curve of the grp tub.
i hope this helps.