Almost any header will bolt right on to your car. The trick is to find some that will bolt on to your engine.
The Custom Cruiser is a Delta 88 from the front doors forward. You'll find that most header manufacturers claim that their headers for the 1968-72 Cutlass cars also fit the 1971-76 Delta 88s. Why do you want headers on that car? It's upwards of 5000 lbs. Even with a 455, it's not a rocketship. You may want to consider the reproduction W/Z manifolds used on the 1969-72 442s. These manifolds have internal dividers for individual runners like shorty headers, are much easier to fit, and won't leak all the time. HP difference might be 10-15 vs headers. You've got a low compression smog motor with a mild cam and small valves, and likely have a Bonneville-friendly rear axle ratio. There are lots of places for performance improvement on that car.
The biggest bang for the buck will be changing the rear axle ratio. A free flowing factory style dual exhaust, a better cam, and an Edelbrock performer intake will make a world of difference.
I only have a cam change in mine and when it kicks out of passing gear, it exceeds the limits of the speedometer so I have no idea what it would do in 3rd gear. A rear axle change would definitely help.
I'd go with the 4-4-2 manifolds, aside from the changes previously mentioned, and cast all ballast overboard. Which means, ridding of unnecessary electrical assist options, replacing them with basic manual types. Rid yourself of those fat bumpers, along with their heavy shock absorbers. When you have all that waste together and pile it up on a scale, you should be surprised, as to how much weight you got rid of. Electric motors can get quite heavy. Rid yourself of the smog distributor timing curve. Replace the centrifugal weights and vacuum advance with those of a pre-smog model. If you have A/C, rid of it, also. Compressors are also heavy ballast
Sweet Jesus ivy I hope that isn't a nice car to start with, only to wreck it. Duals and cam will go a long way. Probably better fuel mileage also. The smog era ford 460s had the timing gear retarded 8 degrees from the factory. I changed mine to a 1970 gear and that alone gave excellent fuel improvements. If you have the smog equipment still intacked do not remove it as it doesnt do anything to harm performance. As was said its the cam, heads, intake, compression and exhaust that killed the smog era engines.
The air pump added was insignificant, as far as horsepower loss is concerned. I doubt that Olds dumbed down the heads to the point where any horsepower loss would take place. Appropriate pistons can offset any compression deficit Why go half way?
All big block Olds heads have about 80cc chambers. Olds varied compression ratio with piston dish, not the heads. The 1973-76 "J" head castings DO have smaller ports and small valves. For a 5000 lb car like the Custom Cruiser, the J heads really aren't a bad choice. This car wants low RPM torque, not high RPM horsepower. A Performer intake and a properly set up Qjet, along with the W/Z exhaust manifolds and a proper advance curve in the distributor will go a long way.
