After a complete tune-up and adjusting the SU's on my stock '72 I am now experiencing a backfiring sound coming out of my exhaust when I decelerate (cruising at 40mph, let off the gas & leave it in gear).

The carbs appear to have the proper air/fuel mixture as per the tip of raising the valve 1/16th of an inch to hear a slight RPM increase and then return to normal. Vacuum tests are showing great results (steady needle 'in the green') and I have steady power under load without a backfire. Idle is steady at ~750RPM and the SU's are synchronized. If I lean the SU's out anymore than I have right now the car boggs down during acceleration.

Any suggestions on what to check next? I appreciate any help you can give!

Decel popping is a sign of a lean mixture. When you go too lean on decel, you will get occasional misfires which send raw fuel into the exhaust system. When this raw fuel hits the hot exhaust and mixes with fresh air, it combusts and "pops" out of the exhaust. Some people tend to like that sound.

Do you know what needles you have?