I would like to regear and posi the rear end in my suburban.
Its a 1984 and left the factory with a 3.08 rear with posi (G80), a dreadfully under powered N/A 6.2L diesel. At some point the rear was swapped for a 3.42 and an open diff, then I came along and stuck the 6.5L diesel in there and attached a couple of turbos to it.

My 6.5L diesel has the twin turbos, 22:1 compression and plenty of low end grunt. Also the manifold turbo is a Holset HE351VE a variable geometry turbine turbocharger and I am building a controller for it, which will give it even more low end torque.
With working VGT and more fuel it will have loads of low end.

I bought a series 3 carrier eaton posi for 8.5'' gears and 28 spline axles some time around 2004 for the camaro, I would like to use this in the suburban now.
I would like to put some highway gears on the suburban when the eaton goes in, 2.73 gears if possible.
So plain is to take this open diff 3.42 rear and remake it with a used set of 2.73s and my eaton posi. If the used GM 2.73 gear set wont work with this eaton carrier then I will go with a 3.08 set or buy a new set of 2.73 gears that will fit.

I figure going from 3.42 to 2.73 gears should net me +3mpg on the highway.
And if I need to tow something more than an empty trailer all I have to do is click it out of OD, 3rd gear with 2.73s will give me about 100 to 200 rpms over 3.42 gears in OD at 65mph.
I think 3.08 gears will make very OD lethargic when trying to tow and towing in 3rd gear will cause the engine to rev a lot more than needed for the light towing I do.
2.73 gears will make OD useless for most towing so I wont even try .

 

The 8.5 GM 10 bolt uses the 3 series differentail for all gears from 2.73 to 5.13, so you can use that Eaton Posi with the 2.73 gears. If I was building this truck with the amount of torque that you are going to produce I would do the 30 spline upgrade. The 30 spline Eaton Posi is the same price as the 28 spline Eaton, and 30 spline axles are not hard to get. This can increase the axle strength by about 30%. I don't know if any of the later GM truck axles are the same length, but all of the 8.5 rear ends were upgraded to 30 spline axles in '90. If you wanted to make it even stronger then swap in a '99 or later rear end. They were upgraded with larger differentail bearings and the ring gear size was increased by 1/8 inch.

I don't quite understand why you bought a 28 spline 8.5 Eaton Posi for a '85 Camaro when it has the 7.5 rear end with 26 spline axles.

 

I remember the V6 was just going to get a diff swap and the Z28 was going to get a bigger rearend.
I think I decided to do an 8.5'' rear end swap with a truck axle on the Z28. Because at the time I had a free supply of old 80s and early 90s half ton truck 10 bolt rear ends.
Around the same time I bought a lightly used 7.5'' Zexel-torsen diff for the V6 Camaro.