WRT linked page: I'm surprised to see DV make such a big mistake.

"the constant vacuum of an SU pulls the air valve open just far enough to satisfy the engine’s air demand. Therefore, at low speed, it is a small carb; at high speed, it is a large carb. This in turn means that you can put a giant SU on a small engine with little fear of having a carb that is too big."

This is only true if... the carburetor has the correct vacuum piston return spring. Otherwise:
If too weak, the piston rises to full height too soon, needle goes to full rich and the taper stops affecting mixture.
If too strong, the piston NEVER rises to correct height, and the mixture will be too lean.

In some cases, there is no suitable spring, and you're left with fabricating and adapting.
Trust me, an H1 on an XK-E or an H8 on a Mini 850 will ruin your day.