Built 273, what sure grip ratio with a A833 3.09 first gear

What are the other ratios in that 69 box

The 3.09 low is only good when the starter gear needs to be in a certain window for the engine to be able to come alive.
And generally ,an engine will like a high starter gear when;
it is of small displacement, or
has low power at low rpm,or you are looking for a quicker take off,or
the engine has to pull a trailer,or
operate at a higher altitude,etc.
Your 265 is getting up there in displacement, and has a better head-design than some others,lol. And it will be getting a turbo, so;
If the turbo starts building pressure soon enough, she may not need or want the 3.09 low. Your starter gear will be 3.09 x 3.54=10.94 So then if it only made 100 ftlbs at say 1800rpm, then the power to the rear axles would be 100 x 10.94=1094 and that is probably not enough to break the tires loose. So the solution is to rev it higher, say 2800, where she might be making 150 ft lbs and the new starter torque would be 150 x 10.94= 1641 and at sealevel that would be enough to, or should be/ could be enough to, initiate tirespin.

But then you come along with a little turbo that makes 200ftlbs at 1800 rpm and immediately pumps 200 x 10.94=2188 into the rear axles and this would be far in excess of what is needed to initiate tirespin. So now the engine revs up so quick in first gear that you can hardly get it into second fast enough. So the solution is a lessor first gear, say a 2.66. Now your turbo only puts down 200 x 2.66 x 3.54=1883 ft lbs, still enough to spin but it doesn't run outta revs as quickly.
All power numbers are fictitious and chosen to prove the point; your results will vary.

Now we get to point #2,the transmission's 1-2 split.
My ratios are 3.09-1.92-1.40-1.00 and the splits are .62-.73-.71.. So whatever I rev it to in each gear, the rpms will drop to those percents. Example if I rev to 6000 in 3.09 first gear, at the 1-2 shift, the Rs will drop to .62 x 6000=3720 If my engine falls off the power curve at 3720 with 3.54s then it will have to fight it's way back up to where the power is to increase it's rate of acceleration. Fortunately for me I have 367 cubes and it is well able. And fortunately for you, you have a turbo. And if engineered right, you will have V8 power, perhaps even more.
Since you already have the 3.09trans and the 3.54 rear gear, IMO, you are off to a very good start. Just try not to get too greedy in the turbo selection. And I can't help you with that as I have no turbo experience...... yet