I think I can help you in a round about way. The Buick manuals are kind of cagey when it comes to identifying the 3 and 4 speed manufactures. The 65 Buick Service Manual for the Skylark Gran Sport states that the 4 speed in the Gran Sport is the same as the transmission used in the 1965 44000 models. The 65 Buick Service Manual for the 43000 and 44000 models lists the 44000 models as Skylarks and Sportswagons. I have a #s matching original 4-speed set-up from a 65 Gran Sport. It is a Borg Warner T-10, and it is going in my 66 Gran Sport Convertible. Since my trans is listed as the same as a Skylark 4-speed, I think the 65 Skylark used a Borg Warner T-10 as well. It also looks like a T-10 in the pictures in the manual. The Ratios listed in the 1965 43000 and 44000 Service Manual are: 4th, 1.00; 3rd, 1.51; 2nd, 1.89; 1st, 2.54. My Gran Sport has a 2.20 first gear and a little closer ratio. I guess Buick figures the 300s needed a little more help getting started. I hope this was helpful.
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Steve B.
GSCA #2173 (1986)
67 GS 525 Stg IV
66 GS Convertible
65 GS HT
63 Riv