Things get tricky quickly because there are not just 800 series and 2600 series cars as we explore things...
The first generation 800 series frame is 5 3/4" long, the second generation frame measures 6 3/4" or 6 1/2" (1930 on wards when the round ends were squared off) long and are also 1/8" wider.
Note: There is the 820 series (not to be confused with an 800 series) that measures 7 1/4" and has 4 wheel trucks.
Note: There is a 650 series that use 6 1/2" frames with four wheel trucks.
Now, getting into specifics - there is no 2600 series per say- it actually is a 2650 series
The 2650 series uses similar 6 1/2" frames to the 650. The prepended 2 to the number generally loosely signifies a 650 series car with automatic couplers upgrades and additional body styles.
Then enters the 810 large series of cars with 8 13/16" long frames.
Add automatic couplers and more body designs and you get the 2800 series of large freight cars.
Now, even more fun, we have the Ives acquisition and all of the lithographed car types to consider and this is why reference books are and so this is why the aforementioned reference books are a goldmine to help sort through this all.
I have seen a few at the local flea market that have the black plastic lobster claw type, with some sort of hook projecting out, and they didn't look very simple or robust.
Those may be Marx 6' rolling stock as Lionel did not have plastic knuckle/claw couplers. Here is a rundown of the Marx Coupler types, not all of which are compatible with Lionel. Diving deeper into the Marx Types is discussed here. Here is a high level rundown of the Lionel prewar coupler types (without getting into the nitty gritty casting and spring and mounting differences)