When a company or person tells you they'll "custom-build" a converter for your car, that's just not true!  There are only so many variables of converter cores, stators, fin angles, and clearances.  If you have to wait for a converter to be built, it merely means that you have a converter combination that's either (a) not in stock at the time you need it, or (b) one that's not commonly used and kept on the shelf by the manufacturer.

The basic core design and physical size of the converter is the base for modifications to meet the requirements. Generally the smaller the converter core is, the higher the stall speed can be.  As you move up or down an inch in core size stall speed will drop or raise roughly 600 RPM. The next most important component in the converter is the stator. The stator is what redirects the fluid flow inside the converter with a degree varying due to fin shape and fin angle, and thus multiplies torque. The two most common stators are the C stator (also known as the hard-hit, or 073) and the CCX stator (also known as the soft-hit or 031). The two photos above show the stators, and you can see how the C stator's fins are more able to redirect the fluid with their surface area and thus give a more agressive "hit". They are either aluminum or steel, with each type having it's proponents. Other converters use hand-fabricated stators made of steel as well to finely tune the torque multiplication characteristics. This is often seen in very high powered combinations, in 9.5" and larger converters. Moving on to fin angle, you'll often see the number after the stator designation listed as anywhere from a 50- to a 325+. This merely is the angle the tips of the primary pump fins are bent to. The more negative the angle, the higher the stall speed. The more positive the angle, the lower the stall speed. You generally want to stay with a neutral or positive fin angle in most race situations, but some street converters can work quite well with as much as a 150- setup, due to their nature. To vary stall speed by about 200 RPM you must change the fin angle about 50. One other interesting note is that the stator will cause a change in stall speed if you move between designs. The C stator will usually have a stall speed 500-700 rpm lower than the CCX stator if all other variables remain constant in a converter.  One other fact to throw into the mix:  the agressive C stator will be a bit less efficient downtrack than the CCX stator.  From my experiences a change of 50 hp will vary stall speed roughly 200 RPM as well.

A critical part in any converter is the sprag, or even the lack of one in a spragless converter.  The sprag holds the stator in place as the converter multiplies torque.  This happens as the converter turbine (finned housing splined to the input shaft in front of the stator) gains rpm and catches up to near the speed of the converter impeller/pump (shell of the converter and fins attached to the rear half of the housing, behind the stator).   Once the speeds near equalization, the fluid flow path actually changes inside the converter, and the sprag now allows the stator to freewheel with the fluid flow.  In a spragless converter, the stator is always held in place, and its slight restriction to the fluid flow path change will cause a bit more heat to be generated, and thus a less efficient converter (as compared to the same converter but with a sprag). 

 As to stall speed, there are some generalizations.  With a Glide car you'll often run the quickest with stall speed 300-500 over the engine's torque peak, or you can also look to the HP curve and set stall speed where you're about 40-50 HP down below the peak.  A 3-speed may prefer a lower stall speed closer to the torque peak.  Again, many things must be considered before making this choice. 

Hopefully this debunks some myths, and gives you a better insight into how a converter works!