The exhaust crossover runs under the carb. All intake maniflods from the 60s and 70s
had them for quicker winter warmup. To work it has to have a corresponding opening
on the cylinder heads to route a little exhaust thru intake. Most newer and aftermarket
cylinder heads don't have the crossover provision because most engines now are
fuel injected and don't need the crossover. It was common on high performance engines
in the 60s to block off the passageway for a cooler more dense intake charge.