I don't know enough about any of teh Bultaco line to make any reasoned response, but why let facts get in the way of a good story....
I'd speculate that it was how they were used, who they were used by. The 360 Montadero was a dual purpose bike that was a development of the Bandido and most likely would be ridden less hard than the race or MX bikes and the extra advance would perk up the mid range and throttle response. It made peak torque at a lowly 5000 rpm, so it was never designed to rev very hard.
The Bandido is a full on MX bike - again a 360 and had a reputation for melting. Much of the blame was placed on the Femsa ignition and poor metallurgy but I have seen no tests of the ignition on a jig or dyno to support those theories.
Neither model lasted very long and it is possible that Bultaco thought that the pointless CDI Femsa was more able to support more advance reliably or they just went too far in pushing peak torque and had too much advance for the motor. It's also possible that they changed the squish design on those motors. Some Bultacos has an offset squish band. The Montadero had a bath tub shaped combustion chamber with two wide squish bands at the sides of the head. Who knows what the squish band thickness was on any of those motors.
That's all based on 5 minutes of Bultaco research and a whole lot of speculation, so take it for what it's worth.
If you have ported that motor and or matched the pipe design, you will probably need less advance than stock even with a higher octane fuel. You have all sorts of scope for a better combustion chamber shape to get the squish band where you ant it in terms of width shape and thickness. Decide on an MSV value you like and design the head to generate that answer. Look at port time are and flow direction as they all impact on trapped volume/effective compression and match the pipe to the ports.
Gear drive would probably be an improvement on chain driven clutches too. In the end, you have what you have, so it's time to optimize that package and start designing a new optimized package for next year.