I thought the 1966 4 speeds were all 8 3/4. Factory had to deal with some many warranty repairs with earlier 7/4”
In the middle of the model year 1966, it became “standard” that all 4 speed 4 bbl cars cars come equipped with a SG 7 1/4”, the 8 3/4” was still an extra cost option. This is true even in 67 small block 4 bbl 4 speed cars, which still had the 273. Only the big block cars got the 8 3/4” as standard equipment.
I parted out a 4 speed 66 Formula S car, it had a 3:23 SG 7 1/4” rear in it. The trans input shaft was melted and blue on the end, it had obviously had a hard life, but the 7 1/4” SG unit was still fine and functioning. I have the paperwork for that car somewhere, I’ll dig it up and see what I can find that may add to this.
