The only proper way to jet a carb is to go to a place with a large box
of brass bits (webers) or needles and springs (SUs), put the car on a
dyno, and run it, change a brass bit (or set of 4 for DCOEs), run it,
and so on. Each run, you look at max power output at a given throttle
setting and RPM range and also look at the exhaust gases and engine temp
and so on.

This is how volvo (or any other car company) did it originally.

Later, when people start buying carb A to carb B conversion kits, like
the weber conversion kit, hopefully those companies did the same sort of
testing. For the IPD weber kit, if it has a "California Air Resources
Board" certification, then they have done this in order to get the
certification which says that it produces a small volume of emissions.
Of course, the "CARB" certificate doesn't speak to how much power you
get, so that may not be the right kit for you. Hopefully John Parker
does work similar when he's putting together a product -- I suspect that
he does, otherwise he wouldn't be winning races or selling supercharged
motors that last past the first full throttle run up a hill in the
summer.

If you've got a different cam, head work, or similar, then all the work
the companies did originally will be useless in your car. You'll have
the choice of either doing what they did or guessing.

Most people guess because getting carbs tuned correctly is time consuming
and expensive. If you're going to guess, get some good advice from
people who have already done something similar to what you're doing,
then get an O2 meter (wide band if you can swing it) and do the tuning
yourself using the road as your dyno.

time=money
performance=money

How much is it worth?
chris