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You really need to talk to the manufacturer of the springs you are going to buy to see what height they will add and then you will know what size blocks to add, the shorter the blocks the better. Keep in mind the factory F350 blocks , IIRC, are 3.75" tall.
Yeah, it sounds like the OP has both the 3.75" OEM block and the 6" Fab-Tech block on the rear axle. Depending on whether you like a "rake" to the truck, you may want more than 10" rear leaf springs.
So the front axle has been lifted 10 inches and the rear axle has been lifted 6.75 inches... which means that the front is 3.25 inches higher than the rear. You need 10 inch rear springs with no rear blocks.
Rezvani's Latest Post-Apocalyptic Monster Is a Ford F-150 Raptor Underneath
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