Some rear hubs are set up different then others as far as spacing goes, I recently converted two five speed freewheels to six without having to do anything to the spacing ( a 73 Schwinn Varsity & a 70's Raleigh Super Course ). Both bikes just happened to have Atom 5spd fw's that I removed, The Varsity I installed a newer HG Shimano FW & the Raleigh I installed a Suntour 6spd Perfect block freewheel & neither of the hubs needed anything done with the spacing & both 6spds work well after resetting the limit screws on the RD's. The one thing that both of the bikes above had that may or may not make a difference is they both use a claw hanger ( like your Fuji does ) on the RD & the axles & spacing may be a little wider then if it is a direct mount RD as I have had spacing issues in the past but not this time. The only way to find out if it will work is remove the 5spd FW and screw on a 6spd FW & see where the lock nut is when you are done ( your LBS could help you with that ).
I looked at my all original ( but very beat up ) Fuji Monterey yesterday & I am willing to bet that a 6spd cog will thread right on without any respacing what so ever, if IRC Suntour specs freewheel spacing wide on most of there 5 & 6spd stuff using their derailleurs which is probably why it will work on this.
Glenn
Last edited by Glennfordx4; 05-01-13 at 05:06 AM.
Reason: added info