is there really no replacement for displacement?...

At a given level of tune, there is no replacement for displacement. The long stroke of the 225 is both its strength and weakness. The long stroke is what gives the /6 the deep low end torque. It also creates high piston speeds which restricts how high it will rev without some extra help. Even though there is factory racing experience with the engine, it was not designed for competition.

However, there are a few things that can be done to give the /6 a little more pep. I have a /6 in the 73 that is my daily driver. I know I can slide a V-8 in if I wanted more power (as I did in the Demon), but I'd like to keep the economy of the /6. My plans are to build my first /6 that will represent my thoughts on the matter.

Make it breathe better.

1. I have already installed an oversize exhaust manifold and a 2¼" exhaust system. It netted additional top speed and 2-3 mpg on the highway. Because I lost carb heat when I installed it and most of my city driving is short trip stuff, city mpg suffered a slight drop.
2. The build up gets an AussieSpeed SL6 manifold and a 390 cfm Holley. I may install this on the engine in the car. It's a little like putting lipstick on a pig without the intermal modifications.
3. Clean up the ports and combustion chamber. This requires pulling off the cylinder head, so it waits for the new motor.
4. The stock cam is okay but restricts air flow. Small engines with low compression are easy to over carb and over cam for street use. When that happens it will actually produce less power and worse fuel economy.

Give it a bigger bang.

1. With the build up, I plan to increase compression into the 9.0:1 range by milling the block and cylinder head. The /6 is great for this as there is no corresponding milling needed on the intake as on a V-8 engine. This is the most radical of the modifications planned, but, IMO, it will produce the most bang for the buck (exclusive of my labor for Dis/re-assembly).
2. Replace the cam with one that will allow more complete filling of the cylinder. Because this is not a high revving engine by design and I'm not going radical on modifications, the cam is going to be in the 265º duration and .450 lift range.

One of the other things that can be done to give the car a bit better high-end response is to go to a lower (numerically higher) rear gear ratio.
Your Swinger probably has a 2.76:1 or 2.93:1 rear gear. Going to a 3.23:1 will improve acceleration across the board with a small decrement in fuel mileage. FWIW, I'm sticking with the 2.76 in the 73 because I want to keep the revs down on the highway.