Most advice is good, but seems far beyond Coryduran's experience or funds. I agree with the advice to enjoy the car as is, plus the currently good mileage.
Many start with advice to upgrade to a 360, then add a newer transmission (lever shift) for direct bolt-up and better torque converter choices, but you also need a bigger rear-end, so custom driveshaft. Of course add BBP wheels & brakes, plus shorten the axles and tub the rear. Quickly becomes a total rebuild of all the car's mechanical systems. The risk is biting off too much and another "project car" that sits for years unused.
To address his initial upgrade idea, a 4-barrel carb can give both better performance and better mileage. However, don't go too big (common mistake). For a 273, I suggest 450 cfm. Even smaller if you can find one (370 cfm), but those are rare and costly. The smallest primary bores you can find will give the best mileage, and vaccum secondaries help, especially for a lead-foot driver.
The toughest part is installing a 4 barrel intake manifold. There are 2 barrel to 4 barrel carb adapters, but probably not enough hood clearance, plus they restrict air flow. You can adapt a later 4 barrel aluminum intake to fit early small blocks by drilling and filing. He will also probably have to adjust or adapt the throttle and transmission linkages, perhaps using an aftermarket cable kit. He will find this initial step will take appreciable time and funds, but is do-able and should bring satisfaction.
My main advice is read a lot and visit ebay and craigslist regularly. I got a new 2-barrel for $25 and a nice 4 barrel for ~$50 by waiting. Don't jump into anything. Many take the easy "buy retail" route via Summit, Jegs, and Year-One, but soon have a $10K car in expenses, but usually not in value.
Re hardened valve seats and unleaded gas, I second caferacerx. I recall a Popular Science article in the late 1970's where they discussed exhaust valve wear with unleaded gas. The engine tests then predicted excessive wear after ~50K miles. The main problem I recall is that the valves would sink into the heads until the hydraulic lifters could no longer correct the gap and the valves wouldn't close fully. Lead oxide particles left a layer that cushioned the valve closings. However, I think StrokerScamp may be correct that the problem did not turn out to be as bad as predicted. Certainly any time you pull a head, have hardened exhaust seats installed, since it is amazingly cheap. You will also be better than Magnum engines which don't have separate seats but rather the metal was just locally induction hardened and tends to crack between the valves.
