Remember a petrol engine is an air controlled / limited engine rather than a fuel controlled engine (like a diesel).
Basically there will be a maximum amount of air / fuel charge an engine can injest and that is controled by things like cam duration / overlap, valve sizes / shrouding, pumping efficiency (such as intack and exhaust mainfolds) etc, etc.
Once you have a carb / intake on that can flow that much air fitting a bigger one will give no more power. Usually the only thing that a bigger carb does is ruin the part throttle and idle stability / sensitivity and make the car a pig to drive.
Any change to the carb will require a proper set up, ie. on a rolling road or engine dynometer, to give any sort of benefit .... which will not be cheap.
My advice is to look at the regulations, talk to other people who run the same engine and find out what you are allowed to do and what works in your formula .... then do that!
What engine is it? Guessing it's either a ford X-flow, BMC A-series, Renault or the old Fiat OHV 1300. Sould be plenty of info on all these out there, if it's the A-series then just ask ..... more than happy to help.
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