[comptiger5000]

It's going to be easier to swap engines. Destroking yours is a lot of work, and without re-tuning the intake manifold, cam and such, you'll lose pretty much all low-end torque, leading to no gains in FE, but a loss in power.

If you want to keep the same car, your best bet is taller gearing. A large engine with lots of low-end torque kept at low rpm will do just as well, or possibly better in some cases, than a smaller engine revving higher to do the same work.