Jeff Smith: Let's start with some basics before we get into the details. For the most torque, you'll want to go with the biggest displacement you can create. But for the best mileage, the smaller engine will be more efficient. Both the 455 and the 425 use the same bore, which means the 425's stroke is much shorter (3.975 versus 4.250). From here, you'll have to decide which is more important to you, economy or power. The engine with the shorter stroke generates less piston travel and therefore better economy, but of course, it gives up some torque. In this case, we'd say go for the bigger engine, since the difference in fuel economy will probably be marginal, and Olds 455s are a lot easier to find than 425s, which were only produced for three model years.
You also mentioned you want to run high-12s or low-13s; we'll assume the 455 is going into an A-body like a Cutlass and not some 5,000-pound boat. To run high-12s, you also must make horsepower. Torque will help accelerate the car, but you'll also need decent horsepower for mph. This will mean not much gear, which is good because you can then use the overdrive to run the engine slowly enough (under 2,000 rpm) on the freeway to get decent mileage. The TH2004R uses a 0.67 overdrive, so with a 3.23 rear gear and a lockup torque converter, at 70 mph with a 26-inch-tall rear tire, the engine should only be spinning 1,960 rpm. Spinning the engine slowly will help mileage.
You mentioned using stock heads. Pay particular attention to a quality, three-angle valve job by changing the intake seat angle from 30 degrees to a combination of 30-45-60 degrees and back-cutting both the intake and exhaust valves with 30-degree angles. Any minor intake or exhaust throat-pocket porting that will help low-lift flow will improve power and mileage. The chambers on these heads are not very efficient, so make sure you have a tight piston-to-head clearance of around 0.040 inch. This will keep the quench active and improve torque, horsepower, and mileage. Piston coatings are nice, but I doubt you'd see a mileage improvement by using them. Compression helps all these as well, but keep it under 9.5:1.
Cam timing has a huge impact on engine performance. For mileage, a wider lobe-separation angle improves idle quality but hurts mid-range torque. Crane's mild cams often use a lobe-separation angle of 112 to 114 degrees compared with 108 to 110 degrees. For the Olds 455 with a 39-degree lifter-bank angle, a Crane PowerMax 204/216 duration at 0.050-inch tappet lift with 0.456/0.484-inch lift would work well. Crane also offers one cam smaller and several larger. A smaller cam helps mileage but hurts horsepower, while a longer-duration cam will help overall power but will sacrifice mileage. Do you begin to see the quandary?
Go with a Performer intake, and we'd suggest using a Q-jet carburetor. The carb flows 800 cfm yet offers tiny primaries that provide excellent control over part-throttle fuel. What you'll discover, John, is that if you can lean out the idle circuit, that will do more to help part-throttle cruise mileage than anything else you do. When the engine is cruising at light throttle on the freeway, it is actually running on the idle circuit, not the main metering circuit. Talk to Q-jet experts like JET or Sean Murphy (SMI) for the mods you'll need to the Q-jet.
Next, pay attention to initial timing, use a quick mechanical advance curve, and experiment with adjustable vacuum-advance canisters. This may give you something like 40 or perhaps even 45 degrees of timing at part-throttle. Don't worry, it won't rattle because there's very little cylinder pressure under those conditions. Crane makes an adjustable-value vacuum-advance can that might help mileage. All this will also improve throttle response and create excellent driveability. It's possible to get 15 to perhaps 17 mpg on the highway with this big motor if you tune the Olds right to the edge.