I'm frankly only responding here because I see no one else has! A Marlin 336 long time collector but such beyond my 'need to know'.
My belief that the external barrel diameter of pre 1980 genre should be the same for a lot of "production continuity" reasons. The magazine tube more of possible micro difference. Perhaps check the rim diameter of 30-30 and the cartridge body diameter of the .35 Remington providing a clue of "possible tube diameter differences.
Across the decades of production, I believe differences over time might far more likely account for production differences such as fore-stock fitment. In a vacuum of 'era' information, a crap shoot. I'd suggest maybe take your question to Marlin Owners Forum.
I don't have mine (all original SC in .35 of "J" serial number prefix) in front of me. But it would seem with yours presumably available to you, easy enough to determine disassembly routine as viewing "in hand"!
Good luck!
John
