If I'm not mistaken (not that that has ever happened before!!) the 53 Special still had the straight 8. I thought the only 53's with nailheads were the Skylarks. If I am correct and you really do have a nailhead, it was a later addition.
Regardless, this is a booger of a swap due to the rear axle and replacing the torque tube.
Pardon me for being a butthead, but I fail to see the need to replace a nailhead with a SBC. Nailheads are really great engines and they are easily available and parts are available through NAPA, by and large. Rings, bearings, valves, cams, etc are not hard to come by and are not particularly expensive.
Keep the Buick in the Buick and show it with pride!!