The "D" rated tire would be the same as a 8 ply rating. Not likely those tires have 8 actual plys, it's just they are rated for that. The "D" rating is a newer system of rating tire load capacity.
1966 Mercury Park Lane 4 DR Breezeway 410 4V, C-6, 2.80 - Streamless.
1966 Lincoln 4 door Convertible 462 4V 1971 Ford LTD Convertible 429 4V Phoenix ~ Yeah it's hot however it's a dry heat!