Why not use exhaust "Y" pipe with 1 cat converter and 1 tailpipe?

Check local emissions for your year car if visual is required. If no visual required, better yet. Just my 2 cents, but you could use 2 inch pipe from "Y" outlet to 2 inch cat in/out and tailpipe just to pass sniff test. High flow cat also not required.

"Hey dont get all defensive now... I wasn't attacking you. Read some ov the posts around here... 'Dope' huh...??? Nice. Gotta love this place."

I agree Dope is a bit strong. But you have to admit that when you can't spell a simple two letter word it does have a tendency to bring your intellect into question.

i think its more ov a sight thing the F and the V actualy tuch eech othur.

...add an "L"...add an "E"...

Do either of your cars qualify for collectors plates? If so, there is no need for air-care.

I have a buddy with a blow through, carbed, twin turbo 91 blazer that makes 600 lb#s at the wheel. He put a single cat on his truck and passed here in WA state. 1.2% & 220 ppm standards.

Pale just likes annoying people with the OV thing.

He has NOT supplied the info we need to help him more (loaded or unloaded testing, idle is a given, is it also tested at speed or RPM, what are the standards, what he's built that may not pass) and has disregarded the people that have explained that cats will not work if the engine has a lot of overlap and is rich.

For example, if the test is unloaded idle only and the standards are pretty high and they do not measure and report O2 levels, a couple holes in the exhaust upstream from the sniffer could do it for free.

Thanks, Pale. Now I know what you have to deal with.
Driving - should be no biggie, if it's not too loaded causing the power circuit to dump.
The NOx shows that anything over about 10 to one is going to be tight. At that RPM you should be "up on the cam". Even my own 340 with a 509 cam would pass those numbers (it was strictly research, I would never run that on the street! )
Idle - this is where you are going to have to balance. 4% is fairly reasonable at 1150 RPM, I've only had 2 cars that would start to lean misfire at that. Both were fairly built. BTW, I'm talking about doing the test, not owning the cars. I did thousands of the tests. However, the HC is going to cause a fit if you have a bunch of overlap.
You mention different engines, but it's a percentage of the emissions, so a pinto with a big cam has at least as much of a hurdle as you in a 440.
HCs are part of the reason they went away from the big motors. A bigger bore has more space to hide. Build a good quench, get a ring package up fairly close to the top of the piston, and use the bleed down lifters (hyd) or go with a roller with less duration.
Dedicated tunes usually are a bit lean and the timing is backed off. TDC is not uncommon. Just don't drive it far that way. Also change the oil just before the test. It will change the CO during the test. If they do not inspect at all, pulling the PCV out of the valve cover (and then adjusting the carb) can also get a few whiskers toward your goal...

BTW. if they are measuring NOx, it's going to be a 4 gas analizer, meaning you can't poke holes upstream of the probe... sorry.

If you are dead set on cats, run an air pump too. Feed them air too so they can burn the HCs.
74 400s had 'em in California, so there are factory brackets and pulleys.

That's a weird test procedure My before you throw money away on a cat system, get the car setup, do some tuning with a wideband THEN have someone run it on a 4/5gas. The biggest problem I see is the idle HC, even @ 1150. Big lumpy cams spew out lots of HC. The Nox test will probably require some retarded timing- this helps the hc to a point too. In CA cats will FAIL the visual on a car that didn't have them from the factory, as will an air pump. Here you have to go to a referee station and have them certify the install and make a new emission tag.