Well, lets look at a few numbers.
(The below info in bold is incorrect because stock rods are 150mm. I wanted to leave it here though because the math is correct for the 75mm crank with 156mm rods.)
In a stock 2.4, stroke is 100mm and rod length is 156 mm. Rod ratio is 1.56.
So at TDC, the total distance between the main journal centerline and the piston wrist pin centerline is 206 mm. (rod length plus half of the stroke).
The minimun distance (BDC) is 106 mm.
Assuming no other changes, to match that 206 mm TDC distance with a 75 mm stroke, you would need a rod that is 168.5 mm.
This combination would result in a rod ratio of 2.2467.
Now you could also have custom pistons made to utilize the stock 156mm rods, by lowering the wrist pin in the piston body by 12.5mm. This method would bring the rod ratio to 2.08.
All of this is important to your question because of piston velocity.
Piston velocity is going to be the biggest factor in determining the reasonable RPM expectations of any particular engine.
The value that we are interested in here is the peak velocity. An engine of the same displacement of another, but having a lower peak piston velocity at the same RPM, will always rev higher than the other.
There are two ways to modify the peak piston velocity. You can change the stroke or you can change the rod ratio.
To lower the peak velocity, you can decrease the stroke, or increase the rod ratio.
To increase the peak velocity, you can increase the stroke or decrease the rod ratio.
Okay, so now that we have that cleared up, /ubbthreads/images/graemlins/dunno.gif , looking at this engine again shows that decreasing the stroke will automatically increase the rod ratio. Both changes will allow you to increase the peak RPM of the motor. This is good for drag racing.
The one thing that you'll need to remember is that by decreasing the peak piston velocity, you are also decreasing the speed at which air moves past the valves at any given RPM. The decrease in air velocity is the underlying reason why shorter strokes yield less torque. And this also means that increased rod ratios will also lead to decreased torque.
Bigger cams will also affect where the torque is made. More importantly, bigger cams reduce air velocity even further at a given RPM, as compared to smaller cams.
With a super light mirage shell, lower torque will be less of a concern once you're moving. However, regardless of the platform, if the air velocity is too low, the engine may never idle.
You'll have to find a balance between being able to idle and chasing the high revving beast that you're looking at creating.