Denny,
Considering the cost of a new 6V4/EZ80 is *usually* well under $20.00 for a GOOD quality NOS tube, I would avoid trying to substitute a 6CA4 in its place. The additional filament current will increase the load on the Power Transformer significantly. Since new Power Transformers for the FM-3 are non-existent, this seems like "false economy" to me.
This is ESPECIALLY true if your FM-1 has the Multiplex module installed. The 6CA4 will push the load on the Power Transformer beyond its normal loading. As you know, heat is the enemy of the FM-3. The additional load of the 6CA4 will make things even hotter.
Bottom line: I would hold out for a 6V4/EZ80.
Make sense?
Bill Thomas