The ignitions you are looking at have a set rev limiter that is always on and prevents the engine from overrevving and getting damaged.
As 62cometman was pointing out, you are looking for an ignition box that has a "2 step" rev limiter that can be switched on and off.
The newest one and least expensive is the PerTronix Digital HP Ignition Box 510. It has some very nice features and has more spark energy than the MSD counterpart.
The previous choice was the MSD 6AL-2
For a clutch set-up we use the Omron Z-15GQ-B7-K snap action switch.
This is the switch that activates the 2 step rev limiter.
https://www.dropbox.com/s/32v634bu8ughz ... 3.JPG?dl=0
It activates as soon as the plunger is depressed but the plunger can be depressed an additional 1/4" before it bottoms out.
The switch mounts to a bracket that attaches to the pedal assembly.
You get to be creative and design and install the bracket so the switch is activated by the clutch pedal arm.
The switch has a 1/2" threaded section so it's depth in the bracket can be adjusted.
It is important that the switch is adjusted so it de-activates as the clutch begins to load the engine.
At launch the throttle is wide open so you don't want the 2 step rev limiter to let the engine go untill the clutch is at least partially engaged.
For The SC'd Maverick 250:
The all in one unit that controls the timing curve and boost retard curve along with a 2 step rev limiter is the MSD 6530. I have used this box along with many of the street and racing MSD ignition systems.
All timing functions are programmable by laptop. There is a timing graph/map for the advance and one for the boost retard.
Once you get lines on map you can just grab points along the line with your curser and drag them to wherever you want to make changes.
Since the MSD 6530 is doing all the timing you simply lock the distributor's mechanical and vacuum advance and use it to trigger the MSD box.