There are definitely 6 cylinder TBI setups right out of GM. The 4.3 engines came in TBI in both cars and trucks and they were equipped with a distributor. The early distributor uses the same electronic ignition system as the carbureted HEI engines with four additional pins to communicate with the computer. There were also 2.8 TBI engines which were six cylinder and distributor equipped. If you can adapt an HEI distributor then you can substitute the 4 pin HEI module for 7 pin HEI module from a CCC equipped vehicle such as a Carquest 55-1541.

At the computer end, if you have an early computer you need to get a six cylinder chip to put inside. If you have a later computer you'll need a six cylinder memcal. Neither is impossible to find.

What's nice about using a computer for the ignition is that you can get exactly the timing you need at a specific load and rpm. It can make the engine respond so much better overall.