In all cases the manufacturer of the kit car should either provide adequate seatbelt mounting points in the structure of the car or at very least guide you in the best and safest method of mounting your seatbelts.
Below are also some points VOSA have on the subject which will hopefully provide a good understanding of what the examiner will be looking for.
Anchorages on Steel Chassis
What to look for:
• Triangulation and bracing will provide strength and spread the loads effectively into the chassis.
• Upper anchorage must be adequately braced to strong areas of the chassis.
• Threaded seatbelt anchorage fastening should be welded into tubes or onto plates of
sufficient thickness.
Cause for Concern:
• Absence of sufficient structure to transmit loads from anchorages.
• Absence of bracing or triangulation from upper anchorage structure.