Originally Posted by FBinNY

You missed the "just about" lead to my statement. Freewheels come in 3 common width ranges. 5s, 6s/7s (7s is narrower and overall f/w width is the same as 6s) & 8s. So yes, an older 5s derailleur will likely have less trvel raange, but they allow for a degree of over travel, and it MAY work if you're lucky.

OTOH - SRAM as a company didn't exist in the 5s era, so any SRAM RD will have travel distance for at least 8s, and more likely 8s. If using a friction lever (not index), that compatibility won't be an issue, except that it's barely possible that the inner (low gear) limit may not be restrictive enough, but that can be worked around if/when it arises.

Indexing makes things more complicated, because the lever, derailleur and freewheel spacing MUST be compatible with each other. FYI 5s and 6s f/w spacing is the same, so if you want index look for a 6s system.

FWIW - sounds like you're working on a sow's ear, so don't waste money trying to make it more that it is. Find a derailleur with adequate capacity, and set it up to work with the friction lever that's there.