Brand new to Thermoquads- a few questions I need help with

You can decrease the size of the rods, they are only soft brass alloy. IIRC Edelbrock rods can be adapted. a smaller diameter rod is richer than a fatter rod as it is less of a restriction in the jet ID. You can also enlarge the jets. There were over 10,000 Carter carb OEM applications, each one was unique in metering. Your 273 is gonna need a lot less fuel than a 340 so youll probably have to fatten up the rod or reduce the jet. an A/F meter will greatly help.