Hi Ponyrider 66,
The kit you are looking at is for '69 and up heads with the 1.75" carb mount bore. Your '66 has the 1.5" carb mount bore. If you go into the on line catalog and check the individual pages you will find that you can purchase all of the necessary parts, individually.
As for the correct carb for your application, that depends on a number of factors. However, unless you are running an after market cam, headers with port divider, the large intake valve, CR over 9.5 and some light head work (at least a three angle), the 38/38 Weber is way to much for your car. This is a syncroness carb with a 370cfm flow rating. It's a high rpm unit. A stock car would not be able to use the flow and bottom end (off the line) torque would suffer. Torque is a function of intake manifold velocity rather than volume. As throat size goes up, velocity goes down. Maintaining the proper relationship between volumn and velocity is no small task. In my mind the best of both worlds is a progressive 2V like the Weber 32/36 (320 cfm - total) or the Holley 5200 (270 cfm - total) where the car runs on one throat / venturi until you put you foot in it, at which point both throttle bores are full open. You get velocity and flow. For our small sixes this is the way to go.
Second item, at $500 the kit is a bit costly. The Clifford adapter is $78 as I recall. This adapter will take the 38/38, the 32/36, and the 5200 (26/27). A rebuilt 5200 can be purchased from Stovebolt for $65. He also sells and adapter to mount a standard air cleaner (Edlebrock, Motorcraft, etc - $25), for $8. Linkage can be made up from hardware store pieces for less than $5. The fuel pressure regulator is not usually needed with the stock 200 pump. So where am I, $181 vs. $500.
Issues. First, you need to be prepared to tune / re-jet any of these carbs to get best performance. There are a number of forum members who have been through the process who will be happy to share their experience if you go this direction. Second, the carbs we are talking about do not support the vac advance dizzy that came with the 66 stang. So, you will need to change distributors at the same time. I recommend the '68 - '73 dual advance unit (vac + mechanical) with a 10* mechanical weight limit and a Pertronix II electronic ignition with a Flamethrower 2 coil. A rebuilt dizzy is $65, the PII is $80, and the T2 is $35, or $180.
If my math is correct we are at $361 for the carb and ignition upgrade, complete. Still well below the $500 for the Clifford kit. Add headers and 1.6 adjustable rockers and you will have completed what Clifford refers to as Stage II, i.e., a 40% increase in rear wheel HP. You will also have a very streetable car.
There's my nickle - Steve
PS: Check this sight and buy the book falconperformance.sundog.net
but let me tell you that after that im gonna get my $400 upgrade just like you described there. 
thanks alot and i will really consider doing that this summer. 
