Considering a swap from 727 to 833OD. 85 long bed 2wd. 8-1/4 rear. Is the driveshaft the same length between these two transmissions?

I can't say for sure but I think likely so. This is based on my 1978, that I swapped a 727 for a 833 from a b-body. I did not have to modify the drive shaft. I believe the 833 and the 833OD are the same length.

This is a truck 833 OD. Same length as a b body 833
I did an 833 od swap into a truck one other time, 30 years ago.
That was a 79 short bed and I used the trans I had in hand at the time which was a short tail version out of an F body.
On THAT One I did have to get a different driveshaft, and I found one with only having to use a tape measure at a driveshaft [censored] at a junkyard, something not often seen in this day of front (wrong) wheel drive vehicles......

As long as both the 727 and the 833OD are the short tail shaft or both are the long tail shaft, they should use the same drive shaft. If one is a short tail shaft, and the other is a long tail shaft, you will need a different drive shaft.

Both the 833 and the 833 OD have short tail shaft and long tail shaft versions, the drive shaft length would depend on which tail shaft the trans has. If both are the same version, the drive shaft is the same, but if one is a short tail shaft and the other is a long tail shaft, you need a different drive shaft.
So:
For a 727, an 833 or an 833OD mix and match:
Short / short = same DS
Long / long = same DS
Short / long = different DS

Both I have are long tail for this one

Since the transmissions are the same length from the trans mount to the end of the tailshaft it would be the same driveshaft.

Ok cool, one less headache to worry about