Hi Jeff, welcome to the Pontiac addiction therapy group
Some general ramblings:
First off, a tranny swap is easily $1500-2500 depending upon the unit chosen, your skills, and a ton of other variables too numerous to list.
You would have to obtain an overdriven transmission (either Auto or Manual) to achieve any mileage gain whatsoever. At most you're talking about maybe 20-30% improvement. This is a huge improvement, but incredibly costly.
The Olds is a decent motor. Headers are good, although improperly sized (too large primary tubes) can hurt bottom end drivability.
I'm not familiar with the specifics of the Olds family of motors.. but the Holley would not be my primary carburator of choice for a daily-driver application.
Specific recommendations:
-Keep Mom, Dad, and the neighbors happy.. spend $100-150 on some quieter mufflers.
-Optimize what you have.. tune, tune, tune.
You will never realize the full potential of your combination unless you dial everything in as accurately as possible.
-Optimizing includes ensuring the Distributor is working properly, including Vacuum Advance correctly attached to a ported vacuum source. The mechanical advance should operate smoothly and at some point, you should have it properly "curved" to ensure the spark advance is being added at the right time.
-Change all the vacuum hoses for new ones.
-Install a Quadrajet carburator which has been prepared for YOUR engine, trans, rear end ratio. Spend some dollars and get the right person to do the work. CliffR provides excellent value.
-Unfortunately, a "650" cam means absolutely nothing to me. I can tell you this, it is very common for owners to install too much cam into their engines, and then wonder why it's good manners just went in the crapper. Lumpy cams sounds great, impress the parking lot crowd, and deliver horrendous performance and economy when mated improperly to a vehicle not suited for it's ideal RPM range. 6500 RPMs does you absolutely zero good. Nice on paper, sucks in the real world.
These are just a few of the things to consider as you strive to restore economically realistic performance to your ride.
To compare, I have a '78 Trans Am with a Pontiac 400. The engine is essentially stock including the poorly designed intake manifold. It breathes through the equally poorly designed cast exhaust manifolds. The carburator is a stock Quadrajet which has been carefully tuned, including proper choke operation. At the time of peak mileage measurement, the filtration was with a K&N, also using synthetic oil, and had been recently tuned and decarbonized.
The trans is a TH-350 driving a 2.56:1 rear end. The cam has lobes smaller than the sweater kittens on Calista Flockhart. In other words, on paper this car should be a real bow-wow. However, it's been carefully tuned to become very reliable and will surprise you with it's strong accelleration.. it will smoke 'em.
Mileage: Documented 22.5 MPG at 65MPH w/o AC on. Your driving habits are probably the single biggest contribution/detractor from fuel economy.