Built in the 70's? Personally, I think you'd be far better served by pulling out the insulation in the attic, sealing all penetrations and seams up there, putting the insulation back in, and then adding more insulation.
Besides the attic insulation, I'd be willing to bet that both your supply ducts and returns leak a lot of air. You might consider Aeroseal--they basically close off all your vents, pressurize the system, and spray in an airborne caulk-like substance, which seals all the air leaks in your ducts. It doesn't improve your home's efficiency per se, but it makes sure that all the air gets to its intended destination. We have a couple bedrooms that don't get enough air, and it's due in part to long duct runs that were poorly sealed when the house was built (in 2006!). Earlier this year, I did a bunch of work in the basement to seal up all the joints that were accessible, and it made a noticeable difference.