I believe the pistons are 15cc dish. Calculations are as follows. I believe the difference in compression height between Vitara pistons and A12 is 3.2265mm:

Vitara Pistons - based on 1.2mm standard compressed head gasket thickness

A12 Crank - 70mm Stroke
75mm Bore = 5.87:1 | 1237cc
75.5mm Bore = 5.91:1 | 1254cc
76mm Bore = 5.95:1 | 1270cc

A14 Crank - 77mm stroke (milling pistons to 0 piston deck height)
75mm Bore = 7.9:1 | 1361cc
75.5mm Bore = 7.98:1 | 1379cc
76mm Bore = 8.06:1 | 1397cc

Thinner head gasket (0.8mm?) would raise the comp up around 8.33:1

Standard Datsun Pistons 6cc dish - based on 1.2mm standard compressed head gasket thickness

A12 Crank - 70mm Stroke
75mm Bore = 8.67:1 | 1237cc
75.5mm Bore = 8.77:1 | 1254cc
76mm Bore = 8.85:1 | 1270cc

A14 Crank - 77mm stroke
75mm Bore = 9.44:1 | 1361cc
75.5mm Bore = 9.54:1 | 1379cc
76mm Bore = 9.64:1 | 1397cc