I see you are right. Below is the confirmation. Not for you but for anyone else on the fence. I do have 31 spline axles. (counted them 3 more times) You bring up a good point about the caliper brackets. I'll call CPP and see if they have any that will work with the Olds rear and their disk kit. Otherwise the modification will get real tricky to work with the kit and over the bearings. I see my axle retainer is not stock.
On a side note, we had an Olds Vista Cruiser when we were kids.
12 Bolt O Axle
The Oldsmobile 12 bolt rear end (O axle) was used under most 1967-1970 Cutlass, 4-4-2, Hurst/Olds and Vista Cruisers. The housing cover can be identified from the pictures below. Internally, the Olds 12 bolt O axle rear end features an 8.5" ring gear. Which is attached to the carrier by 10 bolts (this 8.5" 10 bolt ring gear is not the same as the 1971 and newer 10 bolts). All use 28 spline axle shafts except the 1967-1968 models originally equipped with 3.42 thru 5.00 gears. These use 31 spline axles. The 31 spline models also use larger carrier bearings than the 28 spline models. Consequently, the 31 spline carriers cannot be used in a housing originally equipped with 28 spline axles, and vise-versa without the use of special bearings. 1967-1969 axle bearings are a sealed ball bearing design. 1970's axle bearings are an open roller design lubricated by the differential oil. All O axle shafts are retained by a four bolt flange (not internal C-clips). Three different carriers were used depending on ratio: 2.56-2.78, 3.08-3.23 and 3.42 thru 5.00. These have different pinion centerline to ring gear mounting flange offsets (The 28 spline and 31 spline carriers feature the same mounting flange offset). All O axles use a 1.626" diameter, 27 spline pinion stem. All Anti-Spin models (Oldsmospeak for "posi") are clutch type differentials. Which utilize an "S" shaped spring to provide pressure against the clutch packs.
Thanks everyone for their help. An oddball, but at least it's a stout oddball..