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No you don't. The computer is flexible enough to run a stock 455, if you keep the carb stock. A friend of mine from Maryland had an '81 with a 455 and dual exhaust that he ran on the computer. It got 18 mpg highway and ran 13's in the 1/4. His motor was very stock - 8:1 compression, log-style manifolds, high-vacuum camshaft, etc.
The only reason I had a new chip burned for mine is that I wanted a cam with more overlap and less vacuum. It wouldn't even know if you put a 350 or 403 on it. The only problem I've had is running the tiny fuel bowl on the qjet dry when at wide-open-throttle. It used to stumble and flash the check-engine light at anything above 4700 RPM, until I discovered that it was drying out the carb. I installed an electric fuel pump to run in tandem with the mechanical, and that solved the problem. The electric one pushes whenever the key is set to 'run,' while the mech pump acts as a regulator. I haven't run it dry since doing that. Learned that from my 455-toting friend.
The old OBDI style computer reads O2 levels at the exhaust, and manifold vacuum. It then adjusts the carb to run at a certain fuel richness given those two parameters. If it detects a problem with the motor, or if its in a cold start up scenario, it goes full-rich and dumps fuel through the motor. It needs at least 18 in-hg of vacuum to run properly. Otherwise? Stuff whatever you want in there. That's one of the beauties of old-style computers - they're too ignorant to be the nanny that OBDII is. Post-1996, it became very difficult to swap motors without doing some serious computer tweaking as well. Not so with the old dogs.
A bunch of people eschew all that anyway - pull the motor and all the emissions garbage, put a mechanical carb on it and a new HEI style distributor and you don't need the carb at all. I wanted to keep mine because(a) it works, and (b) I don't know if I'll ever move to a place that has emissions testing. It's still new enough to qualify for testing in many places, and a test in Chicago tells me that it's at least good enough to pass there.
Good to know! Over here in the World’s Largest Outdoor Insane Asylum, they test all the way back to 1976. It is insane. I had to have the TPS replaced in my electronic carb. I can’t do any of the things you mention because it won’t pass smog here.
However, if I could make it look stock then it might pass. It has to pass the visual inspection though. I wanted dual exhaust but forgetaboutit. You can definitely tell there is another tailpipe. Then again, with the Cadillac there was very little clearance for another exhaust pipe unless you go X pipe. No thanks.