1" line wrench on the filter housing, and a 5/8" line wrench on the fuel line breaks it loose. Then pull fuel line out of the way (it's tight but it CAN be done) and unscrew the filter housing. Be careful not to lose the spring behind the filter or (and this is IMPORTANT) the thin plastic seal that goes around the end of the filter housing (this is the only actual seal for the housing). The seal "clicks" in place on the end of the housing.
IMPORTANT TIP: DO NOT USE TEFLON TAPE ON ANY OF THE THREADS. (line or housing)
The Q-Jet is great when you know how to set it up.
And YES, it DOES matter about the spring missing. That's what keeps the filter seated against the housing. Without the spring, you get no filtering.
Yeah, that's a common aftermarket inline unit. You really don't need one like that. They require you to use their replacement parts whereas any inline aftermarket one will work. I used those on my custom applications years ago because they looked cool but they are more money and less filter than many others on the market.
There's only the two different sizes and I would think that all big block engines would get the 800cfm units but you never know. A quick measurement of the primary throttle bore will take the guess work out.