If the trans, pressure plate and clutch are out, measure the machined face of the flywheel and the size of the pressure plate. Its probably larger than 9. 55-57 used a 9.5 but the flywheel was actually machined enough to be able to use a 10.00. GM apparently gave us 6 guys the short shrift and used a smaller clutch but a 10 inch clutch and pressure plate will fit. You might find the same with yours. Four speed truck commonly came with 10.00, some with 11 so take a look and see what the flywheel will accommodate.
