OK, I see them now. That's a synchronized Weber 38 carb, which I have no experience with, but there are a couple of others here who do - I'll suggest you either search for info on it or start a new thread asking.
(as an aside, most folks here don't seem to think too highly of Clifford, their prices seem high and their service doesn't seem to be all that great. I have no opinion, just what I've heard.)
At any rate, regarding carb adapters - you'll need to check out
this page discussing carb adapters, it'll probably answer your questions and maybe some you haven't thought of. If you decide to go with the Clifford adapter, you'll need to measure the distance between the intake manifold bolt centers, since it replaces the Ford carb adapter plate. Looks like that's the measurement Clifford is referring to.
Regarding carbs, if you go with the Weber DFAV / Holley 5200, you'll want to look at
Langdon's Stovebolt, he's got by far the best prices, even though you'll want to either get him to jet it for you or do it yourself.
At any rate, there are quite a few others here with Weber carb of various styles, do some searching and you'll dig up a ton of info. If you hit a snag or need more info, feel free to ask here or maybe start a new thread.