8 3/4 Sure Grip ratio change

You will be able to determine the exact shift points from the power curve.
The 727 ratios are 2.45-1.45-1.00 and the rpm drops are to 59.2%, and 69%. This means that whatever shiftpoint you use,the rpm will drop by those percents.The two different drops means you will have two different shift points.
As to the 1-2
If your power declines very slowly, after peak, then selecting a 6500 shift for first will drop the Rs into second, to 6500x.592=3848, and the powerband requirement is 6500 less 3848 =2652. The centerpoint of that powerband would be 2652/2 +3848= 5174;pretty close to 5200. Now if the outgoing power at 6500 is very different than the incoming power at 3848, then you adjust the shiftpoint up or down until they match. Generally on factory small blocks, the power falls off a little more rapidly at the top the it builds at the bottom. I imagine 6500 will be a little high. Check your data.
I can guesstimate,perhaps 6300 for the 1-2 shift.
As to the 2-3
Here the rpm drop is to 69% so the shift will occur much sooner.Since I can't see your graph I arbitrarily select a number, and do the math. I choose 6100.
6100x .69=4200, for a powerband of 6100 less 4200=1900. And the centerpoint is 1900/2 +4200=5150; gee that worked out pretty close. So again, refer to your dyno chart. If the outgoing power at 6100 matches the incoming power at 4200 then 6100 it should be. If not, adjust up or down until the powers match. Every time you change it, you have to recalculate.
So, the preliminary points are 6500 and 6100, which you will have to adjust to match your curve. Then some shift loops at the track will dial it in.
Now if your TC is stalling over 3850, this will throw a monkey wrench into the calcs

Now having said that, those points are to generate the maximum average horsepower during the run and will generally get you the lowest ET.Those therefore are track shiftpoints. On the street with 3.23s, 6500 will be about 63mph. So basically you just shift it whenever,lol, cuz to run it up any more mphs is speeding. With 533 ftlbs@4000, you're probably gonna want to shortshift anyway, cuz it's like you said, the tires are spinning. You won't get to 60 any faster spinning the tires at 6500 than spinning them at 4000. So save your engine.
With a 417, traction is sure to be an issue.You probably want to address that as soon as possible.
enjoy