Hello! Yes, it's a bit of a tight fit to load the take-up spool on my 1964 635; however, do make sure it's a 120 spool you're trying to fit! If so, then you could try the following:
Place the camera face (lenses) down on something with a bit of softness and 'give' to protect it (I use a mouse mat), with the top of the camera pointing away from you.
With the camera lying face down, open the back and then turn the 120 film winding knob (top right hand side nearest the back, and which has the ASA DIN numbers on it) until the spool drive key is in the vertical position (so its ends are pointing at the front and back of the camera).
Line the slot in the side of the plastic spool up with the winder drive key so that it fits as snugly as it can onto the key, then pull out the left hand knob marked '35mm Film Only' and gently coax the spool past the spigot (if it's still protruding slightly). With a bit of jiggling and gentle coaxing it should go into place. Once it does, then guide the plastic spool so the metal spigot pops into the centre hole - this can also take a bit of jiggling till you get used to it. The spool should now be in place and the left hand knob should have gone back to more or less fit against the side of the camera.
If this doesn't work try taking the plastic spool out again, give it a quarter turn, and see if it will slot into place any better in that position. I hope this works for you.
I find it's a bit easier to remove the exposed film at the end, as the mechanism is spring loaded and this partially pops the spool up once the left hand knob is pulled out.
A couple of things to beware of on the 635: Always make sure it's set to the 'X' flash sync setting. If it's set to the 'M' setting and someone tries to use the self timer it will jam the shutter
permanently. I think the best bet is not to use the self timer, they're usually the first part of the shutter mechanism to run slow or stop completely so I don't think it's worth the risk on an old camera.
Secondly, when using 120 film, make sure the little knob marked 'R' is pulled out and given quarter of a turn to keep it in the 'out' position (see instruction book). It's part of the 35mm film mechanism and if left 'in' it protrudes into the interior of the film chamber and can appear as a shadow at the edge your photos. I hope this info is useful, best of luck and let us know how you go on.