Thanks Tony. I just found out that the collar is supposed to go on after the bell is attached to the TKO. I mistakenly thought the collar went on the retainer first, with the flat side towards the TKO and beveled side towards the engine. In fact, the beveled edge goes towards the TKO.
I'll check the collar and retainer for burrs and run some measurements to check for out-of-round. When I last test fitted the collar, it went on as far as the bevel on the retainer. It did look like some gentle persuasion would allow it to continue. I wasn't sure if this was the norm.