the nominal capacity of a NR RD is to take a 26T largest sprocket but in practice the largest possible sprocket will vary with the installation, in particular
- the geometry of the tab on the gear hanger
- the chain length (because the top pulley moves relative to the A pivot)
- whether you have 5,6,7,or 8s at the rear; the offset in the RD was meant for 5s; wider clusters see the top pulley swinging upwards towards the low gears.
- what kind of sprockets you are using; HG style ones have shorter teeth than (say) UG type ones; this can be worth about 1T more.
- where the axle is in relation to the gear hanger; in the back of long slotted dropouts can be worth 1-2T more
Each 1T requires about 2mm more clearance. Given that the frame has long slotted dropouts I reckon you would be unlucky to have a big problem running a 6/7s freewheel with HG style sprockets.
BTW IIRC there is more than one way to assemble the A pivot; one of these ways gives more chain wrap and a slightly increased overall capacity.
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