Not a piece of cake to tune. It can be done if you have a syncrotester or three vaccum gages, and you make sure the brass jets and needles are okay. There is no subsitute for rolling road work, as the set-up needs to be dailed in to a correct cruise and power CO level.
The stock HS6 carbs are 1.75", and flow about 205 cfm each. Peak power is 106 hp per carb, so in theory, 318 hp is possible, but not with a 200 in a mild state of tune.
There are a variety of main jet and needle types. There are bias swing needles, and 90, 100 and 125 thou jets.
The best option is to use the larger 100 thou jets, and use a needle similar to the OA6 needle used in later aftermarket triple carb kits. 250's seam to like the bigger 125 thou jet, and aftermarket needles such as those used in Aston Martins and Jags.
The carbs look like old Leyland Tasmin or Kimberly or P76 Morris Marina items which were seen with 1750, 2262 and 2600 cc OHC engines.
There are some issues with dashpot drop rate, and the spring rate of the piston spring, and additinally, the kind of oil to use.
These items work very well when tuned by a loving Ford fanatic such as you. After its dialed in, I'd suggest getting an oxygen sensor, and monitor the tune with a volt meter.
One thing with SU's is that they produce excellent low speed torque with good economy, but they hate heat soak and high fuel pressure. The leak because the parts don't have a high aromatics resistance (the English never used unleaded untill 1989, and those carbs are 1970's items), so ensure you use good fittings. Alex has made an excellent sheetmetal cover which ensures the heat from the extractors/header doesn't end up boiling fuel. I'd restrict the line pressure to 3 psi with a fuel regulator.
They polish up delighfully, and the K&N stubbstack and filters need to be used to avoid the air cleaner restricting the airflow and therefore jetting requirements.
