There are plenty who get up in the 20s with their EFI 4.9s. Although, with a full sized van, it's hard to say if that's possible simply due to the size. Dunno.
But I agree, try a simple tuneup, check the computer codes to see if there's anything that may be failing, replace the O2 sensor, and see how it goes.

If not, you're on the right track. Although you don't need to replace the exhaust manifolds. Replacing the stock carb exhaust manifold with the ones from an EFI engine is a very popular performance upgrade.

You'd also need to address the fuel, since the EFI pump is MUCH higher psi than the carb can handle. A fuel pressure regulator would be necessary.