I bought a 73 Suburban recently and noticed it was backfiring on cold start. Check for spark and the spark seemed weak.
then checked for voltage at the BAT terminal of the HEI...only 11.4Volts. Checked the voltage during cranking and it even dropped to only 7-8Volts. This is a common problem with old cars. A lot of stuff runs of the IGN, but upgraded parts (like point to HEI ignition) and old wiring cause voltage drops.
Solution is simple: use a relay to supercharge your HEI directly to your junction on the firewall where the alternator keeps the voltage at max.
http://www.hoigto.com/tech/powerHEI.htm
The cost is well under $5 so a very affordable great upgrade.
result: cold morning starts are great, backfires didnt happen anymore and idle is nice and strong