Make up the rear frame section that carries the wishbones as a separate piece obviously making sure it's square etc then and you might need another pair of hands, clamp it to the rear of the chassis in the right place. Doing it this way you shouldn't have to disturb the main chassis on the table. Once the wishbone frame is tacked on it will let you do the rest although it's over hanging the table.
In an ideal world the table should be longer but if you do it as above it will save any grief moving the chassis and the possible risk of something moving that you never noticed.
