Distributors - Modifying them, how, where, and who 

Terminology - 
Dizzy - Distributor 
When a known engine build formula is used, dizzies can be churned out with the relevant advance curve to suit - as in standard production engines. Reading the 'dizzy working basics' article defines what the dizzy does and how. It's pretty obvious then that any changes at all in the original engine specification is likely to require a different advance curve. This is where most folk fall down when completing the ‘build for power’, by assuming that an alternative dizzy from a sportier or similarly powered engine will do the trick. A simple illustration is folk's preoccupation with sticking a 'S' dizzy into anything tuned in the belief this is the way to go. It's not. It's likely to be as useful as the one you have. 
From the 'dizzy working basics' we can see that altering the advance curve is achieved by changing a combination of the three controlling factors. Starting with the weights, these are by and large left alone - exact alterations to both weights being difficult to achieve. If necessary they can have weight added in the form of blobs of weld, or reduced by filing them. The springs are easy to change, just fit springs with different tensions. Modifying the stop is pretty easy too - increasing the amount of total advance requires removal of material, decreasing it by adding some (weld again). Even the amount of change can be calculated by measuring the distance between the stop and pin, then dividing this into the advance figure stamped into the points cam base plate. 
This is where the modifications are taken out of the hands of the DIY-er. A special machine is needed to assess the changes made. This, and the application of many years experience, is necessary to perfect ignition advance curves 
To start with, it's no use trying to modify a worn-out base unit. The points are the switch, their opening controlled by the cam on the distributor spindle. Fine as long as the points open at exactly the right point for exactly the right period of time. Why is the period of time important? Because it is the CLOSED period that allows the coil to recover to maximum charge before they open again, releasing another belt of artificial lightening. If the period of time is shortened, the coil will not fully recover in time, lowering the power of the next spark. This time period is known as the ‘dwell angle’, commonly referred to as just ‘dwell’. 
A worn dizzy spindle causes timing scatter. After surprisingly low mileage, the dizzy spindle can become quite loose in it’s housing causing it to wobble excessively. This varies the point at which the points do their switching thing, dubbed timing scatter. Experience has shown that scatter of two or three degrees can cause a loss of around 4HP on an average fast road engine, this figure being common on used distributors. On high mileage units the situation can be much worse. The A-series engine is fickle in this area, it’s rapid-burn capability caused by combustion area design wanting precise actuation. This can be alleviated by fitting an electronic ignition switching system (such as the Petronix, Piranha, or Lumenition items). But these MUST be included when the prospective dizzy's advance curve is modified. 
There are now two options - either find a suitable company that can deal with modifying dizzy advance curves, or use an existing alternative. When getting your dizzy modified you need to consider the fact that the prospective company must have intimate knowledge to the A-Series engine and it's requirements over a broad spectrum of applications. So this narrows the field to those that have been actively involved in modifying A-Series engines for some number of years. As far as existing alternatives - these are limited. 
The much-abused 'S' dizzy is probably only really useful in a full race engine when little initial advance is needed, and maximum advance is all in at around 4,000rpm. They're not suited to modern road use by any stretch of the imagination. Apart from anything else - no self respecting road engine should be without a vacuum advance unit. 
This narrows the field further to the only other mass-produced and readily available modified A-series dizzies - the Aldon duo - 'Yellow' and 'red'. The yellow dizzy has a reasonably gently advancing ignition curve to cope with almost anything road orientated. The red dizzy has an 'immediate' advance curve that comes in with a bang and is all over by around 4,000rpm for race use. Collectively they are probably more useful than almost anything else where a 'guess' is concerned. Largely because they have what you could consider as 'safe' curves for their prospective use. Commendably, they do a creditable job in large-bore engines, but are not really the ticket on small bore ones. But, as stated before, they are the only real alternatives available, and are a better bet than guessing/using entirely the wrong one from another engine where no machine is available to check the advance curve. So use them. Anything road-orientated, get one with a vacuum advance. 
For a dizzy that matches engine requirements - this involves either an engine dyno session as opposed to a rolling road session - very carefully controlled parameters are needed here), or one modified by an expert in the field. In the US of A, or anywhere else in the world I really can't help. In the UK, RaceSpec (01925-636959) and Swiftune Racing (01233-850843) are the kiddies. Oh - and Aldon Automotive of course (01384-572553). Send 'em a suitable dizzy with your engine specs - and their educated guesses at what you'll need will be far more accurate than anything else you can come up with 'ad-hock'.