I don't think you'll do any better with a 5.7 than an EFI 5.9. I have an 05 ram 1500 4wd quad cab. It's got 3.92 gears and 275/60/20's, about 33" tall, and it probably weighs in the 5500 lbs with me in it. The best I can get is 19 mpg ish if everything is perfect and I'm doing 60 mph on the nose, kick it up to 70 mph and its 15 mpg. Only modifications are shorty headers, magnaflow y pipe and Flowmaster cat back.

I'm a big fan of the 5.7 but given the challenges of California emissions I would stick with what you have. If you do go with EFI get a custom tune, the stock tune is awful.
Aren't vehicles over 25 yrs old exempt from emissions testing?